Built a treehouse with my boys this summer and it's still holding
Took 4 weekends but we finally got it done. Used 2x8 pressure treated boards, bolted everything into this massive oak in our backyard. My 7 year old helped me drill holes even though he was nervous. Now he won't stop talking about it. We have a little ladder and a rope and it's legitimately cool. Best part is they actually play outside now instead of gaming all day.
Four weekends and a treehouse still standing is impressive. Pressure treated lumber is the way to go. Did your boys help with the actual building or mostly watch?