I have a 6 year old son and we are NC with my ILs. Over the years they have been absolutely dreadful to my DH, and I don't know what we'd say to DC except that they aren't very nice.
When and what did you tell your children? We do have some lovely family on DHs side (extended family) and my family are (mostly) wonderful so DC isn't missing out. They are very perceptive but have never asked or said anything