Two of my grandchildren have grown up just across the street from us. They are great kids and have benefitted from 4 adults in their lives but I have always deferred to their parents in everything to do with their raising.
Sounds like you're doing it right!
Spoil them, and send them back for mommy and daddy to deal with, eh? LOL!
Can you imagine - our only son (so far, at least, more will be coming God willing) is 19 months old, and had ALL FOUR GRANDPARENTS here spoiling him just a couple of weeks ago!