I'd argue the grand parent is in a better position to judge what the child needs and is possibly aware of whats bothering the child.
Bribery and corruption, Christ.
Every situation is different, unless you're specifically involved in that situation you don't know the full story and thus aren't in a position to judge i'd argue.
Don't judge a book by it's cover and all that.
From your given example it sounds more like a case of "child is upset, peer is aware but can't do anything about it and tries to calm child down. child is still upset and peer tries to cheer child up with the promise of something the child will like"
But then it could be something completely different, we don't know. Only those two individuals having that exchange do.
So I'd say, unless a parent is actively abusing their child, mind your business.