Playground rules

Today I was at the mall trying on clothes, so my mother took my 22 month old son to the little play area. They came back pretty quickly because this happened: my son was playing with one of the toys and another boy about 12-14 months comes up and takes the toy from him. My son looks upset, but my mom tells him to be gentle; we've been teaching him to pat things with an open hand to avoid him being aggressive. My son reaches in to pat the other boy and as he does the other boy's mother swats my son's hand and doesn't bother to say anything to my mother or son. Instead of argue, my mother just left the play area. I'm so annoyed that a mother would touch someone else's child. How would you guys handle some stranger putting hands on your child?