I find this really odd, and sad. I've never known any grandchild to be that rude/dismissive of their grandparents. I would have a jolly good chat with your son/daughter and see how they respond. If you're told you're interfering or over reacting then it might be a lost cause.
For what it's worth, my son went through a phase where he was a bit rude/ungrateful towards my mum. He was 8 at the time. I explained how important grandparents are and how they can be such a special part of the extended family. Now 12, he loves Nanna and Pappa and has good respect for them. Maybe the parents need to have a word with their son, and maybe it needs to be you who initiates that conversation?
Just been to a disaster of a birthday meal for the 12 year old grand Daughter
Middle grand kid was being a right pain and - because he is Autistic (a bit - he is fine with us and his other grand-parents) - he was allowed to get away with anything
eventually my wife made a gentle comment and the kids' Mum were ballistic
Oh and the food was over 2 hours in coming in spite of being pre-ordered the previous night - which really doesn;t help when you have kids that have an early bedtime
so the whole thing was a disaster
criticising a parent - even gently can badly backfire
especially the type that can never be wrong
I reckon it will be weeks before we are allowed to see the kids again - and then only if they have no-one else
anyway - that was my day
which is normally the case anyway