Heart_Mom,
One thing to weigh as you ponder this particular option is that Charlotte Mason advocated that a child aged 9 on up be doing his/her own readings if he/she is able. We agree with that thought, as once kiddos are able to read their own material it really does result in better narrations, better comprehension, increased vocabulary (due to seeing the words in print as well as hearing them), more ownership of the learning, and often less waiting on the parent to be the "reader" when the child is ready and able to do it himself/herself.
So, as far as Preparing Hearts goes, with your child being 10 it would be a good idea to be moving that child toward reading his/her own material. We are already expecting the science to be read by the child at this stage, and age is definitely an indicator as to whether the history readings can be done on his/her own as well.
Charlotte Mason also was very firm on a child retelling from a single reading. She did not want kiddos to have heard the story before, as she wanted to focus on the habit of attention during the reading. She also did not want abridgements or young versions of the classics being read to the kiddos, as it would then change their experience with the actual work of classic literature when the child was of the age to hear it for the first time. I tend to agree with her in that retelling from a single reading is an important skill to develop. It also keeps the child's attention better when a reading is fresh and new and not something the child has heard before.
With this in mind, I think it would really be stealing your thunder for future guides to read aloud everyone's material to each other. This would, in theory, be fine for the first year you are doing it, but what happens when the next child gets to the guide and has already heard ALL of the material in that guide read aloud?

To give you an idea of what I mean, imagine that you went to sign up for a literature class at college and you had two choices. One was a class that would cover a set of classic books all of which you had already read several years prior. The other would cover all new books that you had never read in the past. As you were weighing the two options, you were thinking that taking the class on books you'd already read would be easier, because you probably wouldn't have to reread the books or pay as close attention in class and could just get by with doing the assignments. Even if you did choose to reread each of the books, you would already know the plotline and how the story ended, which would definitely impact the reading experience.

The other class would be harder, because you'd actually have to read the books and pay attention to what was going on in class in order to complete the assignments. You would likely be more interested in reading the books simply to find out how the plots developed and how each story was going to end. Which class would give you the better literature experience? Which one would do more to develop the habit of attention?

Which reading of the books would leave you on the edge of your seat?
It isn't uncommon for a parent to have the feelings that you are having as your older child is getting ready to gain some independence. I went through this with my oldest child too. As a parent, we want to keep our kiddos by our side and keep reading aloud to them so that we can be sure of what they are reading, and we don't feel like we are out of the loop.

However, if we keep reading all of their material to them, rather than allowing them to do it themselves if they are able... it quickly becomes ALL that we are able to do with our kiddos as their readings get longer and more difficult. This is because it takes much longer to read aloud anything than it takes to read it to oneself.
At HOD, we desire to allow students to do those tasks for which they are ready and able. This allows us to exchange tasks like reading aloud all of the child's material for other needed teaching tasks like discussing purity, Biblical worldview, self-image, art, music/composers, poetry, etc. These are many tasks that are otherwise skipped because we're too weary from reading aloud all day. We do still hold back storytime in each guide as a read-aloud, so we don't move away from reading aloud entirely. However, we do strive for balance in this area and recognize that your child is maturing and ready to do some of the tasks you've been doing for him/her since she was a young child.
The wonderful blessing is that as your older child moves toward being able to take on more responsibility for his/her learning, it frees up the time you will need to focus on your younger children. Sometimes in our quest to "keep up" with the older child's learning, we forget that our younger kiddos deserve the same time and attention from us that our older kiddos received. There is no way to give those little ones that special time alone with us, if we continue to hold on to all of our older kiddo's learning tasks (especially in areas like reading aloud, which is truly not a "teaching-related" task). Instead, we need to shift our teaching with our olders to things that they truly need us to teach, and there is still much that is needed (i.e. like math, grammar, DITHR, writing, dictation, Bible, hearing oral narrations, discussing poetry/primary source documents, worldview, self-image, becoming a young man or woman of God, etc.).
In closing I'll share that I visited with a mom at convention last year who had been keeping ALL her kiddos together in CTC. She had kiddos ranging from 7 to high school. She was reading aloud everything to all of them, adjusting for each child, and consequently had only made it through week 15 as she was nearing year-end. Instead of having time to do the purity study with her older girls, she just had them read it on their own because there wasn't time to do it with them. She was so busy reading aloud assignments that we never intended be read aloud that she didn't have time for those things that we really intended for her to be doing with her kids. Her older kiddos could have been doing the reading themselves, but the mother was so focused on keeping everyone together because she didn't want to miss out on the readings because she equated "teaching" and "family time" with reading aloud.
Teaching is so much more than reading aloud, which you already know. But I will also share that family-time is so much more than reading aloud together too. This past post will give you more insight on this area:
viewtopic.php?f=6&t=9343
With what you've shared, I would lean toward having your 10 year old read his/her own history and science. I would choose one set of storytime books to read-aloud to everyone together. Typically, I'd lean toward reading aloud the Bigger child's storytime to all. This is because you could choose a different set of storytime books for your LHFHG child when he/she gets to Bigger. Then, you could read the Preparing storytime books just to your Preparing child, which will be a great alone time as that child will be reading his/her other material on his/her own.
I would keep your LHFHG child alone for his/her work to make sure he/she is really understanding the readings and not defaulting to allow the older kiddos to chime in and answer instead. It is also good to read the Thornton Burgess books alone to the LHFHG child as there are so many skills wound within the follow-ups.

I know my little one guards his time with me and loves having me all to himself during his LHFHG time.
I would keep the poetry separate for each guide, because there are so many skills wound within the poetry that the child doing the guide will need. But, I would do the hymn singing from Bigger with everyone. Hymns can easily be sung and enjoyed again and again.
You could also have a "Friday" sharing day, where each child shares something he's done or loved from his guide that week. It can be a poem, a hymn, a music selection from the week, a writing assignment, a section from a book, a drawing, their timeline, a notebook entry, etc. This is a wonderful way for kiddos to compliment one another on their work and for the child to select something important to share.
I'll link you to a thread that you may have already read, as to why we plan for the guides to be done in the way that we do. Perhaps this will give you some further insights to ponder. Link:
viewtopic.php?f=6&t=9344
Blessings,
Carrie