Kid all over my baby 😡

So today I decided to take my little one out on a walk downtown area. This mother and her child walked by and the child came up to my son in the stroller and grabbed his head and was touching him all over. All over his face, arms, neck, head. At first I thought the mom would tell the child to come back to her but she didn’t. I’m really upset at myself for not saying anything the second that child came up to my baby. I just kinda froze. This mom just stood there with her arms crossed watching her child, and I guess my actions on how I would handle it. Idk it was just really weird and I didn’t know what to do or say. It made me so mad that the mother didn’t even try pulling her child away, especially considering the circumstances with social distancing. Like she should know better, right? I’m just so upset about this and mad at myself!! How should I have handled this? What would you have said or done?

I keep replaying the situation in my head and I truly felt like if I said something to her daughter or even pulled the stroller away, she would have said something to me by the way she was just standing there with such attitude. I truly felt like she was wanting to see my reaction.