Few recognise the complexity of child rearing.
Where, if you were going to circumnavigate the world and over there were your parents who already circumnavigated the world, you would ask them for tips, ideas, etc.
Where one has become a parent, one would assume they would turn to their parents for advice.
Many new parents proceed as if they know more than their parents. They don’t want to hear anything and don’t seem to grasp that due to the effort of their parents they are who they are today.
As many new parents have this attitude that they know better all the Grand has to do is step back. Your duty is to observe and say nothing. Not to interfere. The grand kids may come to you to complain and you can nod maybe toss a word, (depending on how old and bright they are).When the grands confide in you, hold their secrets. Never tell your kid anything. In this way you win the trust and confidence of your grands.