Sarah,
If you're planning to give Singapore a try, I would give your ds the Singapore Placement test to see if he is ready to start in 1A. Here is the link:
http://www.singaporemath.com/Placement_Test_s/86.htm
The reason I asked about math placement, is because "Little Hearts..." includes math lessons and activities for Earlybird Kindergarten 2A/2B, whereas "Beyond..." includes math lessons and activities for Singapore 1A/1B. I was just checking to see if math was a factor in which guide you would use with your son.
A plan for all areas of language arts is included in our guides, but moms are welcome to substitute or add-in something else if they already have something they love using.
Some moms who are using Little Hearts... with an older kiddo also add some more formal language arts to "Little Hearts..." to beef it up, because the gentle grammar lessons don't begin until the "Beyond..." guide and then "Bigger..." follows with Rod and Staff English 2 as the primary grammar text.
We do start copywork in "Beyond..." as well and spelling. Cursive writing begins in "Bigger..." and spelling or dictation is also done in "Bigger...". Poems by classic poets are included in both "Beyond...' and "Bigger..". "Little Hearts..." has more fingerplays and rhymes and works on large motor skills with those.
After finding out more about where your dd is at right now, I wouldn't worry about making the big combining decision this year since your dd is not really school-age yet. I'd probably do "Little Hands..." with her when you get to it, taking my time to go through it, and let her ride along for any parts of your son's program that she would enjoy.
After taking the math placement test, if your son is ready for Singapore 1A and he fits best in "Beyond..." according to the HOD placement chart I would probably start there. We've found the chart to be quite accurate in placing kiddos as long as the areas of reading, writing, and math are right on track with "Beyond....". Pay close attention to the area of writing in the placement chart, as he must be able to copy lines of poetry without too much trouble and not balk at copywork. After "Beyond...", "Bigger..." requires quite a bit more writing, so if that's an issue it's better to step back to "Little Hearts...." now.
If you really think your son is borderline between the "Little Hearts.." and "Beyond...", I would go with "Little Hearts..." to give him a chance to grow into some of the skills in "Beyond...".
I would love to tell you outright what to do, but I'm still not sure of your sons skill level in math and writing, which also makes a difference in placement. So, I'm trying to lead you through the thought process that will help you make an informed decision.
Let me know if you have other questions, and I'll be glad to help!
Blessings,
Carrie