By allowing this kids to behave this way you are doing them no favors.

I witnessed a similar situation, and, to be honest, if I was in a hurry I would have ignored it, but my young son was with me, and I did not want him to leave the scene with the wrong idea. So I grabbed one by the collar and dragged him to a security guard. The guard did little, but the other kids ran when they saw what I did. And the kid I had began apologizing profusely.

If their parents complain, tell them not to allow their children to behave in such a manner. They're just kids. It is really the parents' fault.