I hear myself do it all the time. I say things like "be careful!" or "that's dangerous!" or some other warning to my kids. It comes from a good place. I don't want them to get hurt. But I'm hurting them by saying these things.
Self-confidence comes from taking risks. Brains grow with physical activity. Hand-eye coordination develops as you coordinate between your hands and your eyes (intuitive, right?). And... you can see cool things when you're up in a tree.
Instead of hurling warnings, I'm going to ask these kinds of questions: How are you feeling up there? Are you feeling safe? What do you see? Can I support you somehow?
Here's more about this tree climbing thing. Now go find a tree....