How do they let the parents know? Before hand, like, "hey can we do this?" Or is spontanious and the parents are just like "why is my child covered in goop"
They let them know. FBE is a business company, so they have to let parents know. They probably asked their kids to bring some clothes too for this particular challenge.
I really hope you were prepared to clean some of that gunk off. I also hope you warned the parents to take proper precautions, like a change of clothes or a towel in the car.