Home away from home...for the boys

I am hoping to sweet talk my father in law and husband into building this for the boys this summer (adding rails around it of course) They have wanted a tree house for a while and I know they would use it daily. I have a lot of ideas for it. We have a big walnut tree in the back yard near the swing set. I can see it from the back deck. I think it would be a "special" place for them to go. What do you think?