It is rarely aimed at the child. That is what makes it so hard to answer.
He's very small for four, isn't he. We never did it that way, but I'm sure it's fine now. Oh, is she still not sleeping through? Poor you. Every one of them is about you, delivered as an observation about your child, in a tone that makes objecting look like an overreaction to a grandmother taking an interest.
You are not overreacting. You are also not going to win by proving it.
Do not defend the method
The instinct is to explain. The health visitor said. The book says. It is what they recommend now.
Do not. The moment you produce evidence, you have agreed that your parenting is a proposition requiring support, and she gets to weigh it. She raised children and you are reading a book, and in that particular courtroom you will lose every time.
"We're happy with how we're doing it."
Whole answer. Nothing after it.
If she presses:
"I know it's different from how you did it."
Agreeing that it is different costs you nothing and removes the argument she was building.
When it is in front of people
Same words, flatter voice, then move.
"We're happy with it, thanks. Did you see the garden?"
The redirect is not weakness. It tells the room that the subject is closed without anybody having to have a scene, and it lets her keep her dignity, which makes it far more likely to work twice.
When it is about safety
Different situation, different rules. If it is the car seat, the peanut, the walk to the shop without holding hands, you do not soften it.
"That one isn't negotiable. If it happens again, they can't come here on their own."
Say the consequence once, plainly, and then be prepared to mean it. A safety boundary that is stated and not enforced teaches her exactly how much your boundaries are worth.
When she does it to your child directly
You're getting chubby, aren't you. Big boys don't cry.
Answer in front of the child, immediately, gently, so the child hears the correction as well.
"We don't say that in our house."
Then to the child, in a normal voice: "You're perfect. Come and help me with the plates."
The child needs to hear that someone stood up. That matters more than whether she takes it well.
The conversation with your partner
"I'm not asking you to agree that she meant it. I'm asking you to say 'Mum' when it happens. From you it lands as family. From me it lands as an accusation."
Ask for one word. Not a confrontation, not a discussion in the hallway. Just his voice in the room, so the child sees that this is what both of his parents think.
If he will not, that is its own conversation, and it is not about his mother.
When she starts hedging it
I'm not allowed to say anything, am I. Well, you know best, I'm sure.
That is a complaint dressed as a retreat, and it is designed to make you reassure her.
"You can say things. I just get to say no."
Cheerful, quick, then talk about something else. Do not apologize, and do not spend the next ten minutes making her feel better about a boundary you were right to set.
What it will cost
She will feel criticized as a mother, because that is genuinely what is happening, even though you are only asking about the present. There may be tears. There may be a phrase about being made to feel unwelcome in her own son's house.
You will hear, probably from a sister-in-law, that you are sensitive about the children.
Set against that: the alternative is your child growing up in a house where an adult says things about their body and their crying and their sleep, and hearing nobody answer. Children do not remember the remarks. They remember whether anyone said anything, and they file it under what is allowed.
That is the whole reason this one is worth the stiff Christmas.
One story and one script, once a week
No advice you have read before, and nothing we would not say to a friend at our own kitchen table.
The list opens shortly. We would rather have nowhere to put your address than pretend to collect it.
