For 2nd hand, you could try one of the outdoor centres or instructors, they usually replace kit before it's completely ruined, so you get a good bit of use for a cheap price.
For new, Warmbac in Glastonbury make (the best) caving suits. If you've only got one child, who is still growing, buying new is probably not good value though. I've bought a few new kids size suits, but I've got four kids who like caving, so they get handed down and the price is easier to justify.