It goes without saying that I adore my 3 Grands. I take great pride in bragging on them every time I get a chance. You should too.
I had a disturbing brief exchange with a set of grandparents recently. I said to them, I love your granddaughter. She is so sweet. Reply from grandparents...that's what you think...you should have to be around her more.
I am confident this was meant to be a backhanded thank you, and I feel relatively sure that these grandparents love their grandchild also.
So my advice... Say thank you to any compliment. Anything short of that is rude.
Take every opportunity to praise your grandchildren. You have no control over what others say.