Sandra's point about the permanent marker is a good one. How do you decide when a kid is ready to move past just playing with materials to actually learning a technique? My niece wants to help but I'm scared she'll wreck something I care about.
Yeah, I gotta be honest, I see that differently. Maybe it's because my cousin's kid got into 'restoring' an old side table with a permanent marker. What looks cute can turn into a real mess fast. There's a big difference between playing and actually fixing something. That kind of play can ruin good pieces if they get their hands on the wrong stuff without someone watching close.
I used to worry like @sandra_lane6 about kids ruining good wood. My view changed after starting my grandson on a scrap wood birdhouse. He learned to test stains on the bottom first, and that focus translated to his schoolwork. Now he helps me fix small things like picture frames, and his patience has really grown. Those Saturday projects taught him more care than I expected.