I had one of these and it was fun for about 5 minutes. very quickly got quite boring as no real pull to it. (cheap though)
(edit with correct url) We traded up to one of these which is loads more fun and he (5yrs) can fly it easily on his own. (not so cheap but miles more fun.
We also have a 2.5m flexi rage for when we want dragging down the beach is moderate winds.
2nd hand is a good idea (thats how i got my range) but smaller kites are harder to come by 2nd hand in my experience. As with most things, you get what you pay for. I didn’t mind spending a bit more because i like flying them so even if the lads loses interest ill still use it. Ive nearly come to sell mine a couple of times but know id regret it so end up keeping them.