Well, what I usually tell my kids, if they were moving around underneath is:
“MOVE or I might fall on you!”
“Get out from underneath me. I may fall on you!”
If they are too disruptive:
“I am trying to concentrate. Take your fun elsewhere!”
If you are really annoyed inform the staff that you will not tolerate children in the boulder area because it is not safe. It will be their job to check and inform the children.
We all started somewhere! We have to exercise a degree of tolerance, yet duty regarding their safety.