My experience with my own kids and, more recently, my grandkids, agrees with Hector's that your proposed distance is definitely too much, especially for the 7-year-old. 5-6 mile days, yes, but definitely not more than that. Unless, of course, you want this to be absolutely the last backpack they will ever be willing to go on!

On the other hand, it will be a marvelous experience if you can get your stepdad to slow down a bit for the sake of the kids. The trip should be planned around the kids, not the adults. If you can do that, and if the kids are in good shape before you start, it could be a really fun trip!