Sometimes, but mostly only when they complain about how hard they have it, because one works late sometimes and how are they meant to cope with their kid for three hours twice a week or something. Then I think they don't know how good they've got it and get jealous. But then I also think how sad it is that they don't see the good side of just being one parent and the kid(s), for the relationship.
SWK holidays are perfect for not being the gooseberry in the room (mixed metaphor, but you get my drift). And many a 'traditional' family kid joins in the single family kid fun, and 'traditional' parents wonder why we are having so much fun.