Depends on where you are going, if your heading south then a 3mm will do, depending on your tolerance for cold (and the weather) you may get away with out one. The most important thing with getting a wetsuit is to get one that fits properly, if it's loose it wont work and if it's too tight you'll find it difficult to move in. For the 10 year old I would suggest buying it nearer the time to ensure it fits properly.
Fortunately modern neoprene is a lot more flexible than years ago which makes getting the fit right a lot easier.
You might want to consider a rash vest as well (definitely needed if you go without a wetsuit).