Forget the cheap plastic dollhouses that look like junk after a few months of play (not to mention, there is too much plastic in our landfills already). Build one of these sturdy wooden dollhouses that your kids can play with for years, then fight over who gets to hand it down to their own children one day.
This is a little more advanced project that does take several tools, so you might want to read the plans to see how you can stagger your project over 2 or 3 weeks. That way you can just pick up the tools you need as you go.
For the plywood sheet, remember that many building supply stores will cut wood for you in-store. You can get your plywood down to a manageable size that is easier to transport and work with.