Ok, I don't know why people are steering you away from the 4Runner. How much crap do you need to take with you on these trips?
We regularly go off the beaten path in our home state of Tennessee AND take cross country road trips to visit family in our 4Runner. Check out
my thread for how I organize everything.
My kids are younger than yours (my oldest turns 8 in a few months), but man, there is ample room for kids in that second row. We have a 3rd row as well, but for adventure trips, we use the back for cargo. The third row comes into play when family is in town visiting or when one of my kids has a friend over, and we end up taking them all to a park or something.
Think of the places you can comfortably go in a 4Runner that you won't go in a Highlander, minivan, etc... Just seeing the looks on our kids faces and hearing their squeals of delight when we drive through a creek or slowly make our way through some completely non-maintained, rutted out, muddy service road to a great view or secluded camping spot is absolutely worth not having an extra 1" of elbow room.
Get some
Plano trunks for the back, a Raingler net to keep all the extra stuff in the back like sleeping bags or backpacks, and a
rifle case to put tools or muddy hiking boots up top, and get out there.