Speak up let her know she’s overstepping her boundaries and as his mother it is you or his dad’s job to discipline or not and a child shouldn’t be popped for doing childlike things grandma or not.
Your husband needs to set some boundaries, whatever they are or whatever you want done, that's his mom.. I know when I had issues with my MIL , I told my partner and he talked to her and things have been great ever since.
Don’t let things like that slide she had her opportunity to raise a child and here’s yours. If that’s not the way you want to raise ur baby let that hoe know no over stepping you as a mother. Don’t care what her title is