I've finally got two grand children in the game, they are still children. The children are cousins. Two brothers married two sisters, and the grandparents are the brothers parents. The sisters don't have in-game parents, I created that family as five kids living with the grandfather, but I made the parents first, made genetic offspring, made the grandmother, then deleted the parents (so can't remember what they look like, didn't save them at all). All five of those children looked very different as teens, looks and body shape. One of them looks really weird as she is really skinny with a small head and long neck.
I will have to compare the kids with the paternal grandparents and see if I can see any resemblance.