Some random 2 year old was pushing my sons head and my instinct was to shout, how would you have handled the situation?

So I just got off work, picked my 2 year old son from my sisters place and off to the park we went. I let him roam free around the playground, and I sat on the bench while I watched him. He was putting sticks that were on the steps through the little holes and a little girl was just watching him. At first from a distance, I thought maybe she doing some sort of gesture with her hand, and my son looked fine from behind. Then I saw the gesture again. So this time I got up and followed by a third gesture. To my surprise the little girl was pushing his head back! And he was just taking it and I think he even ended up looking the other way kind of in sadness and silence. I shouted from across the playground, I didn’t care if other parents were there. I shouted, “Hey! Stop that! Don’t push him!” to which I think she responded, “ok.” I grabbed my son made sure he was alright and went to the other side of the playground. Another dad asked me what happened and was relieved that the same little girl hasn’t pushed his 1 year old and some months.

I was so appalled with the parenting of whoever’s child that was. The dad told me that the little girls mother was sitting under the tree, and I looked over and she was just on her cellphone. I didn’t want to confront her because who knows if I would go off on her or what, plus I was trying to have my son have a good time and was eventually going to do our 3 mile walk as soon as he was done playing. What a goddamn joke though. She wasn’t at the playground supervising her child and wasn’t able to tell her what was right and wrong. I understand I’m probably making a mountain out of a mole hill here but I was genuinely irked by this. My son is a very loving human being and loves to hug little babies and cuddle with them (We see this all the time at family parties)

What would you have done in the situation? I acted purely upon instincts.