Two of our very best friends are the parents of 5 year old twins. Our friends know all about our 'quad' and are the most amazing support anyone could ask for.
They haven't really explained any of our dynamic to their kids, it's not something that they need to know. They just know our additional partners as 'friends' and that's working out fine.
Our friends wonder, however, if any of their discussions about 'grown up' things have perhaps reached the smaller ears in the house...
'Dad' was watching some random youtube video of a woman and his son noticed and asked him if that woman was 'Daddy's girlfriend'. 'Mom' answered, 'No, Daddy doesn't have a girlfriend, he has a wife - Mommy is his wife'. To which the child replies 'Oh, lots of people have girlfriends and wives'
We all had a good laugh about that one