This happened to me a few times in my twenties at friends’ family parties. My own family (by that, I mean my mother) would never do this. Partly because she thinks it’s rude, and partly because she gets everyone to sit together — there IS no kids’ table. :)
I remember as a child at my grandpa’s parties, we kids always sat with the one or two single women at the party. I always thought they were so cool and it was so fun to sit with them. But now I look at that and find it so sad. Why did they sit with us when my single uncle was ALWAYS at the adult table?