If you never want to be invited back to your kid’s house, just offer those words of wisdom.
Indicate that the fancy private school is a rip off or that over attention is smothering the grandchild and they have to let the grand kids off the leash sometime.
As your child knows more about parenting than you do, (shelve the illogic) as s/he has to ‘prove’ this, unless you really never want to visit again, open your mouth.
Most Grandparents, (who have sense) can sit back and watch the show without taking it ‘personal’ without believing they have some ‘duty’ or ‘responsibility’.
The best action is to be as bland as possible in front of the parents, hold the secrets of your grandchildren, and make them know they can not only trust you but get worthwhile advice from you.
In this way, you ‘please’ your child, (the parent), you make yourself beloved by their children, (your grandchildren) and can enjoy stress free visits in which you actually have some input into the future.