I grew up only knowing mum's side of the family and have no issues with it. Dad won't talk about his side of the family at all. I actually still don't know what happened there, but I know it was after mum and dad were married. I think his family hated mum and he told them where to go.
My point is, you don't have to tell the kids anything. When they're old enough and curious enough they may ask- I do remember one night mum sitting all is kids don and giving a simplified version of why we don't see dad's family, but it must have been before I was 8 because I remember it being in the old house before we moved, so I actually can't remember what she said.
And- I never felt as though I missed out, either. Probably had a far nicer family life because of it