I think best to measure yourself and then call them, apparently they are very good and it's what i've been advised to do and shall be doing to get mine! I've been told to call treehouse.
Im a size 10 and got it fitted at HOYS last year. I'm an adult extra small with short back but I have the laces done up as tight as they go so I would say you would prob need a childs large as I think the child XL and adult XS are much of a muchness. Would definately reccommend a quick fitting though - treehouse are great