Playground Etiquette

Yesterday morning we spent a bunch of hours at the hospital. Since getting there is such a journey I try to multitask as much as possible when there.
There are many things I DON'T like about the government run children's hospital that attends to my daughter's needs, but there are also a lot of wonderful things about them.
One of the more superficial "likes" is that they recently finished a HUGE construction project that includes a completely new facility for all the medical appointments equipped with (drumroll) really great playground equipment created in such a way that it is for the exclusive use of patients, is safe, and surrounded by transparent walls so that mom is on the inside waiting to be called in to see the doc but has full view at all times of her child. They also made it accessible for children of all capacities. This is slowly becoming less of an issue for Mic, but when they first opened it, it was huge! This was the only playground equipment I'd found that she could actually use independently!
So, here is the question of the day.
What is the correct manner in which to deal with someone else's rude child? For example, what would you say to the small girl talking to Mic saying, "Girl, Hey, GIRL! NO! You can't come up here with us." While her grandma sits complacently on a bench clear on the other side of the area and probably can't even hear what is being said...and doesn't look to friendly herself.
Really, dear reader, I'm all ears here, and dying to know!...what would YOU do?