Many grandchildren are fed up with their parents.
With you there, your own kid is stressed. She wants to ‘prove’ what a great mother she is. But your grands don’t comply. Somehow they know what’s happening.
Grand kids seem to get the fact, (if you know how to play the Grand Game) that their mother/father is trying to prove something, and they are not going to comply.
When you do nothing, they see a confederate.
They come to you.
You DO NOT defend the parents, especially when they are wrong. You don’t repeat the stupid; “They are your parents and know what is best.”
Shake your head, shrug. Best answer.
In this way, your grands will confide in you, trust you, and when you tell them something, they will listen.
But keep this secret from your kid.