It sounds like your younger dd will do great with LHFHG! I'm so glad she will get to do that - we loved doing the 1 year sweep of history LHFHG gives first. As Charlotte Mason would say, it gave our dc "pegs" to hang their subsequent history upon.
Not to mention it gives a solid start as it's also rich in the Bible. When first beginning to teach CM style skills to my dc, it felt like it was easier for them to learn because they were first learning these important new skills with a subject matter they were somewhat already familiar with - the Bible - an excellent start for introducing LA CM style skills.
When choosing between Bigger Hearts and Preparing Hearts, you are right on track thinking about writing skills.
Dc truly do need to be able to do everything on that placement chart to start a guide.
This past thread that compares the writing in BHFHG to the writing in PHFHG may be helpful as you choose:
Amount of writing in Bigger vs. PHFHG:
viewtopic.php?f=6&t=3602
Also, printing off each of the 1 week samples of BHFHG and PHFHG and comparing the 2 will help. Keep in mind PHFHG's "I" boxes are meant for the student to do completely independently, and the "S" boxes are semi-independent and meant for you to teach a bit and then for them to finish on their own. The "T" boxes are teacher-directed. Also, in PHFHG students begin to take over some of their content area reading, starting with Science. They read their science on their own, complete the experiments and lab sheet follow-ups on their own, as well as their science notebooking. This is in keeping with Charlotte Mason's wise philosophy of once dc are able to read on their own, they need to be doing so more and more for their schooling, as when we read something ourselves, we retain it better.
There is SO MUCH that dc gain from this independence! And HOD really holds their hand and eases them into it. My dc are thrived with this, and continue to grow in their academic skills by leaps and bounds each year with HOD!
Here are links to the 1 week samples, so you can check them out:
Bigger Hearts:
http://www.heartofdakota.com/pdf/BHFHG-first-week.pdf
PHFHG:
http://www.heartofdakota.com/pdf/PHFHG-sample-week.pdf
Your child's LA skills of reading and writing are important when choosing between BHFHG and PHFHG. For PHFHG, dc need to be reading well independently, ready to write creatively, ready for short written narrations, beginning to be quite adept in oral narrations, ready for dictation and daily grammar including diagramming, comprehending what is read to them and what they read well, and able to follow clear step-by-step directions independently. Some 8 yo's going into PHFHG can do this, but most of those 8 yo's can do it because they have done BHFHG first. That is NOT to say that an 8 yo coming into HOD will not be able to do PHFHG, it's just that you will want to be sure he/she can.
Let me encourage you that doing 2 HOD guides is NOT hard! We do 3 and love our homeschooling days together!
Not to mention we are finished in a very manageable amount of time.
When dc are accurately placed, doing multiple guides IMO is far easier than combining dc who are so far apart in skills that it is not in their best interests to combine, nor mom's best interest, as mom will be tweaking or skipping things all of the time. HTH, and I'd love to hear back from you about your oldest!
In Christ,
Julie
I looked at some samples of the curriculum and got really excited about what I saw! It's so simple and non-threatening (HA!) and I love how poetry is included in the daily lessons. We tried My Father's World for my daughter in 2nd grade, and it didn't really fit us very well. How is this curriculum in relation to MFW? 
If you get the chance to check out the placement chart and share about each of your dc with the first page of the chart in mind for sure, we can give different placement options for you to ponder and help answer your questions until you come to peace with your decision. 
I totally respect Julie's opinion. She always gives wonderful advice. She was actually one to advise me on doing Beyond with both of my boys, based on our unique situation. Every family is different for sure! Plus, this is our first year back homeschooling after the last two in Christian school.