DTXplorer are not especially cheap.
Problem with most drum kits is they’re not sized for 8 year olds; if you want an acoustic kit rather than electronic, I’d go for a “jazz” kit with a smaller bass drum (18in if possible), so the kit can be set up more compactly.
I have an Alesis electronic kit as well as my acoustic kit and I think you could set it up so all parts of it are reachable, but I’m not certain
The only other alternative would be to buy a “junior” kit but he’d probably grow out of it in 2-3 years and you’d then be stuck with a tidgy kit you can’t get rid of, and the expense of another kit all over again
have a look on http://www.drumwright.co.uk or http://www.drumshop.co.uk
Oh yeah, you’ll need cymbals as well, and they’re not cheap either