Put the armour on the girls and a good number on them will either stop playing, or never start. That is what my daughter believes. And she was a collegiate AA and played on USA. She is 5'6" and weighs about 125. She has no interest in hitting or being hit, and is convinced the level of contact will increase with helmets. She has also said the number of times someone's head makes contact with you when you play is fairly high, glancing you when going for ground balls or ducking a head while trying to drive past a defender. Things that aren't going to cause injury but there is contact nevertheless. She has zero interest in getting hit with a helmet every time those things occur.