If the parents pay the girls as a token of hakoras hatov, and each one gives according to what they feel (besides for a fixed minimal charge to cover basic expenses), it would seem that the money is not included in the issur of giving a present on Tisha B’Av, and it may be given even on Tisha B’Av itself. Once the money is being given as a present and not a wage, it is not longer subject to the Chazal that “the money won’t see any berachah”.
However, such day camps/playgroups should only be run if there is a need and it helps the parents fast. If, however, the parents are able to manage without such help, and the only reason it is being done is so that both the children and the girls helping have what to do, then it’s difficult to allow one to run such a thing, especially Tisha B’Av morning, when it causes one to take his/her mind off mourning over the destruction of the Beis HaMikdosh.
(The above write-up is based on a Teshuvah written by R’ Yom Tov Sanger in Madanay Yom Tov)
