One of the first rules of building children’s play structures is to countersink all exposed hardware. For good reason. If you fall and slide along a post, you might get a scrape and some splinters, but you’re much better off than if your kneecap hits a protruding bolt on the way down. Follow the countersink rule for all kids’ treehouses.
Speaking of splinters, take the time to sand rough edges as you build your house. Your kids and guests will be glad you did. Also keep an eye out for sharp points, protruding nails, and any rusty metal.