To start off with, I cut a box from Saltines in half and gave one to each of them (gotta make sure they are equal to keep the peace!)
A box from saltines will perfectly fit 2 graham crackers vertically. Using a box underneath helps provide a stable foundation for building your walls so that you don't have to sit there holding walls waiting for icing to harden.
Speaking of icing, use my recipe for The Best Royal Icing. It will harden quickly. Feel free to skip on the extract since you won't actually be eating this icing.
Let the kids have free reign at decorating their houses and see what their imagination and logic comes up with. My son decorated the one at the top and my daughter decorated the one at the bottom.
But to offer you one more tip, save some Halloween candy to make these houses. Kids get too much Halloween candy anyways. I just take all the stuff that is colorful and stick it in a big ziploc bag and hide it. There is always more than enough candy that they don't even miss it.