Kathy,
I apologize for taking a bit to pop back in to this thread. I appreciate your patience during our busy season.
I have a few ideas for you to ponder as you consider where to place your kiddos. As far as your 10 year old goes, if he is between Preparing Hearts and CTC on the placement chart, then I would definitely lean toward placing him in Preparing Hearts. It takes a very independent, strong reader, strong writer, who reads a long series of directions well to flourish in CTC as the first HOD program. If the child fits squarely in CTC on the chart, then the child will do well in CTC. However, the parent will still need to train the child in using the CTC independently for the boxes labeled 'I' as the child won't have been trained in those skills within the previous HOD guides (as other user have been who are coming out of Preparing Hearts instead). I would have your son do as much of Preparing Hearts on his own as he is able, making sure to follow up with him right after his readings with any scheduled activities and discussion.
In having your 10 year old do DITHR 4/5, Rod and Staff 4, and the creative writing lessons within the Preparing Hearts guide you should have a good fit in the 3R's. If you using a different math that you enjoy already, then there is no need to switch to Singapore.
As far as your 8 year old goes, I would definitely place him in Beyond (in order to give him another year to grow up in the area of writing). Do copywork as required in Beyond daily and do the writing as scheduled in the guide. A year can do wonders in boys, not to mention if you lessen the battle over writing, your days will flow better! You will also use either the Emerging Reader's Set or DITHR Level 2/3 with your 8 year old. If you desire a gentle introduction to grammar, then do the grammar lessons as scheduled in Beyond. If you desire a more rigorous grammar approach, then add Rod and Staff's English 2 to his day. Simply continue with his cursive handwriting.
Even if you have already read one of the spines within Beyond, I would encourage you to consider repeating it within the plans that are in Beyond. In the long run I think that your placement will be more accurate if you take out of the equation the fact that you're already read one of the books within Beyond. The skills woven within the Beyond guide, along with the Biblical application, and hands-on activities will make it feel totally different. We had to reread books we'd already read too, as I wrote our guides after my oldest had gone through those time periods already!
For your 6 year old, it sounds like Beyond will be too big of a stretch which leaves him in LHFHG. One option that you could consider, if he is a late bloomer, would be to have him sit in with LHTH with your younger child and then add the 3R's from LHFHG to it for his K year. For your 6 year old I would plan on adding The Reading Lesson for phonics (with CD), A Reason for Handwriting 'K', the Do It Carefully/ Finding the Answers 2 workbook set, and the Earlybird math with the hands-on plans in the LHFHG guide. You would essentially be following most of the right side of the page from the LHFHG plans, and then having him sit in for all of LHTH for the Biblical knowledge, fine-tuning of skills, and playful feel. Then, the following year, you could head into LHFHG with him at full-speed, doing the grade 1 options for the 3R's.
This would allow you to focus on doing Preparing Hearts and Beyond, while not neglecting the important skills of teaching the 3R's to your Kindergarten student.

Just something to ponder. Otherwise, you could begin LHFHG at half-speed doing it over two years instead.
Blessings,
Carrie