Ah yes, I remember going to the dump with my Dad. He would go there to dispose of trash, I'd go there to pick up discarded radios and TV's. Sometimes I'd also get something with an interesting motor like mixer, drill, or sewing machine.
I had tons of fun taking them apart, removing components, etc. I learned a lot about how things were put together and identifying components. Kind of like what Dave does in his videos, but from a 7 to 10 year old perspective.
The only problem is that giving a 7 year old a busted old TV, radio, or computer might not receive the same enthusiasm from the parents.