What would you do in this park situation?

you bring your preschooler and baby to the park and you sit on the grass and play with your baby while your 3.5 yr old runs off to the play structure. 2 little girls (about5,6 yrs old) then start being really nasty to your kid and sticking out their tongues and saying he can't play here and just being little brats. The mother is standing right there and sees whats going on but doesn't care and continues talking with another adult. Your son is getting a little mad but doesn't understand why they are mean and keeps trying to play. What do you do?