We don't update the Autarch blog as often as we should, which probably means folks don't check it. I suspect the afterschool ACKS class will provide a fertile source of posts, and wanted to call your attention to what's hopefully the first of many to come: http://www.autarch.co/blog/dont-roll-1-kids
This is great.
So, once they are done building and buying things, are they still gonna go adventuring?
Eventually they will run out of money, so I'd guess so. I have been having them track their character's ages - something the ACKS character sheets leave off, alas! - to make sure that after spending everything old age will put a maximum limit on their ability to say "OK now we hole up in our domains until we have another 815,000 gp."
My two are (very slightly) older than your group, but … that is pretty much a spot on description of what I’ve experienced with them. It’s been a little humbling: I don’t remember being this good at just playing with the imagined world at their age.
I think that kids today have much more experience imagining a world that's not ours than anybody ever has before. My son is never more than a few feet away from some set of tools for pretending to be an elf. It's surprising how true that is even for non-gamer households, and how many of his friends' parents turn out to have played D&D or read the Simarillion.