I think your reaction is understandable, I mean, not all grandparents know how to approach kids.
I grew up really close to my paternal grandparents, so even if my grandpa seemed severe or grumpy I knew there was more inside, and that he had a quirky and kind side to him. My little cousin, who had been living in another country, didn’t know that when she met him for the first time, so the poor kid got very scared and avoided him; my grandpa didn’t know what to do and, while I could tell he was a bit hurt by that, decided to give her space. However, a few years later, when they met again, she came to adore him too.
(Sorry if telling my story was too off-topic, but the topic of grandparents always warms my heart.)
I didn't grew up with grandparents so I don't know if they're this fun. Once, my grandma visit, but there's this sort of barrier, and I'm just a 12/13 then, so even though my grandma will approach me, I shrugged her off. I know feel bad for what the 12/13 year old me did.