Child actors are rare, expensive and annoying. And they have a horrible tendency to grow up. Unless you are aiming at a audience where a child will be a major attractor you don't want them.
And is unusual for doing so, much less married parents with three kids.
It is the rare exception that highlights the rule.
Karl Marx was opposed to the family, so his progs are too.
It’s not just science fiction — in any form of entertainment families and especially marriage is either non-existent, failed, or so horribly dysfunctional that almost any other state looks preferable.