Not all grandfathers can develop a good relationship with young children. I was an only child, and my father was nerdy and bookish. He had been OK with me as a child, though not really a "fun" father - always wanted to over-explain things if I asked a question. He never really related to his grandchildren - by the time they were born, he was old and in poor health and found them too noisy and boisterous, when at the toddler stage. He did try - would read stories to them occasionally, but couldn't really figure out what what appropriate to their age.
I would have liked him to have had a better relationship with his grandchildren, but had to accept that was the way he was.
LynnK - some of the suggestions from other posters are good and I'd suggest you try, but in the end it's possible that you might be like my dad and find it difficult.