S.D.
Do they have to do the fast pitch. I know a lot of the teams around where I live use the big 16 inch ball so it's not as hard. Would they be able to switch to that. I do agree that a child and adult hit differently. I would not let the 10 year old play if it was me unless they changed the ball. I guess the other question also is where in the field is he playing. If it's so right field, and not many people hit that way, maybe you can let him play, and tell them that when the pitch to him, it has to be a slow. they can still do the wind up, but they can adjust the speed, so it doens't look like he is getting special treatments at all.
I would still vote for not palying though.