How to ask her nicely

MK

So i live near a huge park, everyone from all over the city likes to go there when the weather is nice to do all sorts of things with their family, and to take walks and exercise.

I go there with my family and our dog, i have a rare breed here so managed to connect with this old woman who also has the same dog breed and i see her there sometimes walking her dog. I'm not friends with her, i don't know her name but i see her sometimes and we exchange hellos and chat for a minute or two about our dogs and we go on with our day.

My main problem with her is that she touches my son's hair, every single time. My son is under 2, i take him with me in his stroller and he has a curly hair which is something that you don't see here often, he is always the centre of attention everywhere we go because of his long beautiful curls that get him compliments everywhere we go, the thing is nobody tries and touch it, they respect the space.

She is always touching his hair, during those 2 minutes there is a full minute of her hand in his hair, and I want to bring it up next time in a nice way. How do i phrase it in a nice way that won't offend her?, specially that she is my mum's age. She comes from a different culture and this is something that could be normal to her.