Get 3-5 bolts of various sizes, the corresponding washers, and the corresponding nuts. Let your child figure out which ones fit together and practice screwing on the nuts.
I started with 3 of them for B. I wish I would have started with just 1 until he got the hang of it. He had a hard time getting the nut started at the top so he got frustrated. After he got the hang of it, it was really fun for him. They do sell plastic nut and bolt kits for the same purpose, but this was cheaper and the real thing so he could be “just like dad”.