How to deal with mother-in-law

How do I kindly tell my MIL to STOP feeding my 4 week old baby cereal or a little bit of jello mix that is watered down in his bottle(I give her plenty of breast milk in bottles so he has enough to eat) she keeps telling me to do it because he will sleep longer(feeding him baby cereal)yet I know it’s not a good idea, she has done it multiple times when he is over there for 3-6 hours and it’s beginning to piss me off to where I’m not going to allow him to visit if she keeps feeding him stuff that I don’t agree with, she even keeps trying to get me to give him watered down apple juice, I don’t know how to politely tell her to stop since she is kind of a very out going person and can be mean as fuck.