sweetkeys,
You've already received great advice, so I'll just pop-in to agree. The recommended age range for Preparing Hearts is ages 8-10 with extensions for ages 11-12, so we wouldn't recommend for a 7 year old to do Preparing.

It would be a much better route to do Bigger Hearts with your 7 and 9 year olds, as you won't be pulling your 7 year old behind for years to come that way!

At HOD, when looking at combining, we find it easier to correctly place a younger child and beef up for an older child as needed.
We follow-up our Beyond Little Hearts program with Bigger Hearts as they are two different approaches to the study of American history. Bigger is completely biographical, so your kiddos will learn about the men and women who were important in American history. This gives a completely different feel from what you studied this past year through MFW.
Blessings,
Carrie