Anytime a child boy or girl gets a chance to play a sport or just screw around in the back yard is great, not just baseball but any activity, teaches them to be with others , they learn it is ok to have fun in life, also they do not fell left out, for any child that wants to play a sport there should be a spot for that child up to hs age boy or girl or physcially challenged. as for baseball the biggest thing that made me a good hiter and used it in college as a player and a coach and even in the minors is wiffle ball. You here people say it will mess up their swing bs if you can hit a wiffle ball with the way it breaks and dips and can even make it rise or knuckle you will be a good hitter , we played in hs and in college took part of bp, and to make it even better you can go to the wiffle golf balls and hitting will really take off and so will hand eye cord.
all children should have some physical activity no matter what their skills are, none should ever be left out. and also none should ever be forced let them choose what they want to do.
And FW if you mean how much throwing, a child cannot ever throw to much it builds arm strength, just don't gas every throw, long toss, half speed to work on hitting target chest high things like that you can never hurt the arm by throwing right just stay away from curve balls, wiffle cb is ok ball does not put a strain on the elbow and shoulder tendons.