The Pri Megodim (306, M.Z. 4) is mechadesh that even when sechar Shabbos is forbidden, if one gives the money as a present then it’s allowed. The Mishnah Berurah (s.k. 15) and Aruch HaShulchan (s.k. 12) rule accordingly.
Based on this, the Shu”t Be’er Moshe (5:103) suggests, that if one adds on slightly more than what the girls ask for, then it’s clear that the money is being given as a present and not for their work and it would be ok to give it on Tisha B’Av. Based on this, it’s said in the name of R’ Elyashiv (cited in Toras HaMo’adim pg. 161, ois 27) that if one gives money to the girls who ran the day camp as a present and not as wage for their time, then it would be allowed and they won’t be subject to the curse of “not seeing any blessing”.
However, this would be good if the parents didn’t agree to paying a certain amount before sending the child, then we could say that it’s being given as a present. However, in most cases, girls fix a price beforehand, and the parents agree to it, and they have a right to demand the money – if there is a fixed amount and they can demand it, how can we call it a present?
Even if we are lenient and allow them to work (even though it’s likely that they will take their mind of aveilus) as it is a mitzvah, they still won’t see any berachah from their work. Therefore, it would seem that the best thing for these girls to do is, they shouldn’t fix a price and they should simply tell the parents to pay an amount which they think is appropriate. They may fix a minimal price to cover their expenses, however, for anything extra, they should leave it open and ask the parents to pay on their own accord.
