Wonderful project Miller. The kids'll love ya. But trees expand radially and if this isn't accounted for will damage/deform the trunks or pull apart the tree house floor and supports. Been there done that (starting out as a 'free-range' youngster with hammer/nails and scavenged boards more than 1/2 century ago). These days I cobble together tree-supported work tables at Crown land campsites but the underpinnings are kept an inch away from the trunks and are secured by overly long lag bolts that can be backed out as the tree grows. It's true that kids outgrow tree houses in short order (hey what's 5-10 years in the scheme of things (in the life of a tree or when you're 60+)) and if you're good about all this you'll take the whole thing down the moment the kids outgrow it.