Is there a way to try out both and see which one the child prefers? Maybe the clubs have some open door events in the autumn?
I did karate and judo around that age. It was mostly about games and having fun. Not really about the techniques etc. Definitely not about self defense or things adults relate these kind of sports to.
I have been lucky enough not to be on the applicant’s seat often, but I’ve interviewed others. Few times someone has asked for feedback, and it is basically always the same (I will just find nice words to express it):
There is only one opening, and someone else did slightly better than you in the interview.