Just remember..... if Mama aint happy then nobody is happy. Make Mama happy first. If Mama no likey then it's a no go. Your house your rules. We don't have to serve EVERY single aspect of childhood when they are under our roof. Even if it IS normal then this normal can be done somewhere else. You don't have to have any other opinion then you don't like it so it's a no. No fuss no muss. Just a no will do.
The jammies will need to be fitted pretty well so that's there isn't a lot of give at the shoulders. She may try to pull her arm through the arm hole and put it downwards if they are big on her. You should't have any problems tho cuz the current available footed jammies pretty much run slim (narrow from armpit to armpit....... around eleven inches for size fours) The new jammies are flame and fire retardant and are made to fit snuggly.
If you don't get this stopped you are most likely gonna have a humper. If she gets too much "consulation" with rubbing with her hands she will most likely move to rubbing herself on furniture. That's what's coming so deal with it now while she's young enough to learn that it's okay not to soothe herself THAT way.