Has this been discussed elsewhere - it must have been.
But, it just occurred to me, reading some Stonewall rubbish in an article. If you remove the terms "Mum and Dad", what do children call their parents?
They can use their names for sure, but that's a bit harsh and unloving in a young child. Do they call them Birthing Parent 1 and Birthing Parent 2? Lacks a bit of emotional context I feel. Or perhaps Person A and Person B? It all becomes disassociated, non-emotional language. They might as well be robots.
In discussing your parents with other kids "my parents" is of course universal, and can be used for Lesbian and Gay parents. I think it is pretty standard, in those cases anyway to have two Dads, or two Mums. Never been a problem.
So in the home, what is Stonewalls suggestion for say, a four year, to call their parents.