Well, you have two options here, dear.
Either you pin a sweet smile on your face and let your MIL's comments about nappy cream and vests and such like go in one ear and out of the other, and only react to the big stuff, like interfering when the new pram was mentioned,
OR
You tell her you are sick to death of her interference and WILL SHE PLEASE MIND HER OWN BUSINESS!
This will relieve your feelings for a minute or two, but cause a full scale row between you and her and between you and your partner, who apparently sees it as no part of his duty to you to tell his mother to button her lip.
So, love, next time she annoys you, take this piece of advice. Say nothing until you have fastened the baby's nappy and put him back in his pram, then walk out of the room to where you keep your coat, muffle your head in it, and say the very rudest word you can think of.
That done, pin that sweet smile back on, and go back to MIL.
If she starts off on the same track again, smile and say, "Sorry, I haven't time to discuss it right now, I must get the dinner on, will you peel the potatoes for me, please?"
Or if it is nowhere near dinner time - "Just going to put a load of washing on. Could you start the ironing for me?"
That'll get her out of the house! With all those bairns I am sure you are never without washing or ironing, and even if you never iron anything there is no reason why MIL shouldn't do your ironing for you, is there?