They are meaningfully different scenarios, though.

If you and your partner board the plane, sit separately, and one of you sits next to me that's not a negative for me. You'll sit, you'll watch a movie, read a book, whatever. You're self-contained.

If you and your five year old child board the plane, sit separately, and your child sits next to me that's a clear negative for me. Your child needs attention and assistance. It's bad for you, it's bad for the child, it's bad for me. Probably also bad for whoever sits next to the parent because they’ll be standing up and sitting down constantly to go and attend to their child.

I get that it isn't "fair" in a very straightforward examination of the scenario but take a step back and it's just making every passenger's experience more miserable in an attempt to gain more airline profits. If it happens just watch, the airlines will introduce a "guarantee not sat next to a solo child" add-on fee for you to pay.

At which point a homo sapiens specimen becomes a homo economicus?