The Eleph HaMagen on the Mateh Ephraim (585:23) cites a number of poskim who are melamed zechus on those who take money for mitzvah jobs that they carry out on Shabbos, such as chazonus, tekias shofar etc.
Although they don’t see any berachah, they need the money to pay for bread, and the curse was never said about such people. The curse of not seeing any berachah was said for people who are rich and are looking to make money, for simple poor people who have no money and simply need money to buy basic food, Chazal weren’t speaking about, therefore, it’s ok for them to take the money.
However, the above limud zechus won’t help in our case, as most girls that run such day camps are not supporting families, they simply want to make a bit of extra money.
