We have a lovely step grandchild aged 5. Known him since he was 18 months. He calls my husband and I by our first names, but he's a bit special and we feel our names should reflect the lovely relationship we've got. However, he's got his own grandparents too (one set he hardly sees). Our daughter (her husband is the dad of above) is having our real grandchild in 5 weeks and we'll be Grandma and Granpy (or Grandad, not sure yet).