Motherjoy,
The ladies are doing a great job of helping you talk through possible options, which is such a great help!

Often in talking through things, you can find what works best for you somewhere in the midst of the thought process that goes into the conversation. With that in mind, I am going to add my thoughts to the mix for you to ponder as well. Even though I only have 4 kiddos, I have always worked part-time to full-time while schooling my boys (which has always made for very full days at our house). For the last 15 years, we have juggled running our family business while I've been writing and homeschooling and all that entails. My husband worked two jobs for 10 years of that time making scheduling school in a timely no-nonsense fashion very important.
As the years passed, I discovered that knowing exactly what each child was to do each day and having them able to do what was being asked of them was huge. Tweaking was a time waster for me, and working to combine kiddos who did not readily fit into a guide together left me exhausted. Having each of my boys in their own guide with its own expectations was a true dawning for me (and a life changer), as each kiddo had a special placement specific to him. With a family of your size, obviously there will be kiddos who will fit into the same guide, making that a definitely workable part of your plan. But it is good to realize that forcing a fit that isn't there means more planning/tweaking/adjusting for you. I also realized as the years passed that it was MY time that was precious during the school day and that everyone needed ME! This meant that having the kiddos have to come to me to constantly adjust and tweak assignments took my precious teaching time. Or, it took my nights, weekends, and summers to plan the changes. When we stopped tweaking and just moved through the guides as written, there were less interruptions during my time, as my boys knew what they were to do and could do it (for the most part).
This is why, to me, correct placement is so key. When kiddos are well-placed the need for me to tweak is removed. This means more time available for me to do other things (and it means less interruptions and negotiations during the day). If kiddos do not do what is planned within an HOD guide, due to too much tweaking, and then try to move on to the next guide... often they are unable to do what is asked in the next guide. This becomes a continuing pattern, until the guides are finally tweaked so much that the plan falls apart.
With this in mind, I would encourage you to take a look at the placement chart with fresh eyes to first discover where each child truly fits on his/her own (on the first page of the chart mainly). Pretend that you are brand new to HOD, and approach placement in that way. Once you have discovered where each child fits on the chart by himself/herself, then you can begin entertaining possible combination scenarios (always keeping in mind that it is very helpful if kiddos can do what is being asked of them in the guide without adjusting, as otherwise the independence becomes much less which will require more of you.)
I would also encourage you to consider whether keeping everyone on a similar time period in history is a necessary goal. I used to think that it was, and I spent quite a few years working toward that end. But, I must say that when I finally let go of that goal and started looking at history as something that is enhanced by having kiddos studying varying time periods, I had a huge sense of relief in no longer needing to force that fit into our school structure. Instead, I embraced the fact that my boys were studying history integrated with the Bible and with church history. I was thrilled simply that God was a part of our history study and that my boys saw history as "His" story. These days, I am amazed at the depth and richness of our dinner conversations whenever they turn to history as everyone has something to offer from his own studies (and my boys are rarely on the same time period). When I let go of the history time period being the constant, the tweaking lost its importance too. Instead, I was free to focus on the skills that each child needed, without adding another layer of trying to make the skills fit the family history cycle.
I know for me that each year, I am relieved to know we can just move on to the next guide and that I don't have to rethink everything every year. This would be the case with your older 4 kiddos if you went to placing them where they truly fit on the placement chart. I know different families approach HOD different ways, and that is one of the blessings of homeschooling. But, I also know that if kiddos cannot do what the guide is asking them reading/writing/volume-wise, then it requires more of me as the teacher to continually modify the plans. Since there is often not more of me, I train my boys in the design and needed skills in their guides to gain the independence that helps me be a time-conscious teacher. The bonus is that the skills they are being trained in are useful for life!
At this point, from what you've shared thus far, I'd lean toward your oldest in the new American Guide, the 10 year old in CTC, the 8-9 year olds in Bigger, the 5 year old in LHFHG (probably half-speed) or combined with the 3-4 year old for LHTH (as the helper and adding in the 3R's for the 5 year old). I would look at the Bigger Guide being your focus for the upcoming year in training that pair of kiddos in the skills needed for success. I believe the 8-9 year olds could potentially do more of Bigger independently as the year progresses, helping you out time-wise there. I think they might be able to eventually read their own history and science, and begin reading from the guide more too as the year progresses. I had one of my boys do this more, as he could, and it was a tremendous help to me at the time! I have a thread somewhere on the board about doing Bigger more in that way, if you're interested.
Anyway, I do think that down the line this set-up has potential to make your days better. Putting your time in now to train the 8-9 year olds would be time well-spent as your next set of kiddos coming up are not into full-blown "school-age" yet. Separating the 8-9-10 year olds would also allow for a bit more rotation of kiddos in playing with the littles, as your CTC child could do playtime with the littles while you are teaching the 8-9 year olds... and your 8-9 year olds could play with the littles while you are teaching CTC. Anyway, just some thoughts I had that may help as you ponder options.

I pray you will find the Lord's peace as you seek His best for your homeschool. He has gifted you with many children, and I know he will gift you with a plan for this coming year as well!
Blessings,
Carrie