Mary,
The moms are giving you such helpful advice.

Now that you have the guides in hand and know that you actually can do more than one HOD program at the same time and still have time to live your life.... I think the most important factor to consider is whether or not your 7 year old will be ready for "Bigger..." next year. If not, then it would be best to stay with the history for "Little Hearts...".
Either way, you sound like you're moving in the right direction. Placement can be interesting at first as there are so many factors involved to consider (especially when trying to decide whether to combine or not). However, like water2wine mentioned, once you get on track with the correct placement, moving on to the next program becomes much easier.
Once your kiddos get older, it also becomes easier to combine them for history and Bible, if needed, as they can listen and comprehend much better as that part of their thinking develops. So, combining when they're younger can actually be harder at times than combining when they are older. So, even though they are in separate programs now, there may be some combining possible again for some of them later down the road.
Just thinking out loud with you!
Blessings,
Carrie