Ask yourself three questions:
1 is she very lonely, or a person with low self-esteem? (this seems likely to me)
2 How would you deal with the situation if she was your mother, not your husband's?
3 Have you other child-minders on hand?
If she is lonely, or has low self-esteem, she is trying to make herself feel more important by this behaviour. You don't have to put up with it, but she won't stop it, unless you tell her politely but in a way she cannot misunderstand that it annoys you.
Wives frequently feel critising MIL would come better from her son. Usually, husbands have no desire at all to tackle their mother about anything. If yours will, by all means let him do so. If you would not accept this behaviour from your own mother, than say precisely what you would tell her to MIL.
If you have other child minders on hand, then use them and limit MIL's time with your son. When she asks why, say with a smile, "Oh, you have made it so clear we were inconveniencing you, that I made other arrangements."