I have been with my wife for 6 years. Her kids all had children of their own while we were married. They are still little with the oldest being 9 and the youngest 3 months, then 3 & 4.
The oldest asked me one day whether he should call me grandpa and I told him he can call whatever he wants but if wanted to call me that he had to ask his parents. They all call me by my first name. I feel it would be a blessing to be called Grandpa. I love all of them and when they come over, they are so excited to see me and ask for me. The parents and my wife call me and the grandchildren best friends when it comes to parties or holidays, and when my wife talks about them, she always calls them my "grandkids" or "grandkids".
I have been around when the little ones ask how many grandpas do they have and they make sure they know my name does not come up. I know as they get older they will get more curious, but it does hurts to feel like "the guy" that married mom or grandma, even to family or close friend.
I feel like I'm not part of the family, but I will lay my life for those kids. When they come over, I always go out of my way to entertain them and play things they love, superheroes, cars, catch, hide and seek, build a BMX ramp etc. When the kids are bothering them, they tell me to play with them and they know I will, because I love them with all my heart.
How should I express this to my wife?