I had my 10 year old grandson over today. We completely assembled a bike from my son's old Felt road frame and (mostly) spare parts. The only things I had to buy new were a couple of black headset stack spacers, handlebar tape, and a pair of low cost tires. I traded with a BF member for a black water bottle cage but it hasn't arrived yet.
I let my grandson do as much of the work as possible and he's a good student. He was quite curious about how the derailleurs work and asked some pretty astute questions. The whole job only took us around 3 hours which I think is pretty good for his first bike build. We took some side trips to the bike shop for tires, the hardware store for electrical tape and we also took Mrs. Grouch to lunch.
When his mom came to pick him up he told her that he had a good time. I hope that he had as much fun as I did.