Shelley,
Normally, we wouldn't recommend backing up in our programs, as one guide does a good job of preparing a child for the next guide in line. However, you have some good reasons for desiring to consider doing Bigger Hearts with both of your kiddos.
Bigger will be a step back in the level of history readings, however the Extension Pack will help take care of much of that potential problem. The activities in Bigger Hearts will definitely seem younger than the projects in Preparing Hearts, however if your kiddos like activities then they will most likely enjoy those in Bigger as well.
The science is simpler in its readings in Bigger than in Preparing, but again the Extension Pack will extend that area. You could most likely use the math, dictation, and grammar schedules from Preparing Hearts for one more year for your 4th grader just using the next scheduled level up in each area from this year. You'd also move up in DITHR with your 4th grader.
One concern I do have is whether the following year you would desire your younger child to skip up to CTC with your older child. I think that jumping from Bigger to CTC would be too large of a jump for your younger child to do successfully. So that's my thoughts.
Blessings,
Carrie
