In reply to ranger*goy:
Corax is a great workhorse, I've had a few, I always keep the old ones cos they're so useful if you take out friends - they go small enough for bigger kids but fit a lot of adults as well.
But they're very, very heavy for a 10 year old, also if he doesn't fit a corax he definitely won't fit one of the others ( corax is about 24"). I've waited for kids to grow into stuff a few times and waist is probably the slowest thing compared to feet and height.
Can you extend the elastic on the rear of the leg loops if that's the problem? If the leg loops are too close to the waist (belay loop too small) you're a bit stuffed.
A good alternative then would be a clog centre harness, massively adjustable but not padded.