Yes, most orgs use children to prove their non-profit bona-fides. They conduct 'educational' activities, you see. And it will be detailed in their filings.
They don't tell you that they're using you. Just like they didn't tell anyone they were getting a significant sum per child per man hour for pulling the accursed grass.
Kids thought they were getting a field trip to the beach, then found out they were there to do the work adults didn't want to do. And someone kept the money. None went to the schools, did it? None went to the kids, did it?
It's been going on for years. And the adults leave feeling so fulfilled. Look at all the good they've done.