You are a really fucking cool dad for even wanting to build a tree house with your kids, good on you. Literally my best memories are with my dad building cool shit like tree houses, which eventually snowballed into me having a million random skills and the two of us building actual houses together. Even if you can’t build something as complex as a treehouse right now, it’s the participating and engaging with your kids’ lives that really matters. They will remember you as the dad that showed up