Regardless of who says it's ok...if one of the kiddies gets an eye shot out you're in for a Class 5 Hurricane Shit Storm.
Don't do it. Don't take part in it. Don't council/advise them (other than erring on the side of safety and caution telling them that they should supervise their kids). Don't condone, facilitate or support them.
If they're worried about their kids hurting themselves...then they should supervise it. Why? Because they are THEIR kids...not yours. Full stop, no ifs ands or buts.