Decathlon sometimes have childrens shortie wetsuits. They are not always in stock (like most things there!) so it's worth a ring before you visit a store. The last time I saw some in Nottingham, there was the size you suggested. At the price they were, I bought one for my daughter at her current size, and one at the next size - £12 each! She's very thin and so wears a furry and oversuit over the top to keep warm (both from Daleswear)
Good luck with the search.