In reply to rug:
The sizing on rock shoes is all over the place, even the kids stuff, ebay is a bit of a gamble.
I've even found that the sizing differs between different pairs of the same shoes.
If you buy big at least they can grow into them, but if you can get them at the wall to try them first so much the better.
Keen kids tend to go through the toes quite quickly, as they find the easiest way round reachy moves they can't make is a double handed pull accompanied with frantic toe smearing, depends on the surface and how often they climb as to how soon they go through them.
Once you try to get them small adult shoes rather than the bulky toed kids shoes it gets even worse.
It's difficult to persuade kids to use lace ups and if they're too young it's a pain to tie them yourself but you can reduce the volume with laces that you can't do with velcro. If you can persuade them to wear laces they can edge better and their footwork improves.