haven’t done it, but I’d try a rain sensor. and if the child is into music, a guitar tone knob.

I don't have any kids but if I ever do, I'd get them one of those spring and wire based 30 In 1/etc kits. That's how I fell in love with electronics.

I originally learned with an old Radio Shack electronics kit like this one:

https://old.reddit.com/r/electronics/comments/g6vyjc/old_ele...

I tried to get my kids interested with the modern equivalent, Snap Circuits, but they didn't take to it. It was too limited and toy-like.

I later acquired an old electronics kit like the above, and they were much more interested in that. You can still find the older kits on eBay for not too much money.

Keep in mind they may not take to it like you did. My kids' own interests went to more mechanical stuff and small gasoline engines and electric motors instead.

Don't steer their interests, feed them. Provide breadcrumbs for their curiosities.