I agree with Ingrid-that's the best way to determine what size will be a good fit. However, if you are unable to do that, I would say 6 " of ease is a comfortable amount of ease for a child. That's enough for plenty of freedom of movement.
That said, if the sweater is to be worn over another shirt, I would give it more ease-in that case 8" would be a good amount.
Good luck with the sweater!