You don't have to engage or justify staying in your seat, just say no thank you and end the conversation
I remember as I was annoyed that this whole thing was holding up my flight. Family asked someone to move, they declined, family kept insisting. Boarding line was getting held up due to this. FA arrives, starts imploring the man to move his seat, obviously just trying to get boarding complete so we can all move on with our lives. Eventually the man got up & changed.
(to be clear, I don't do this personally and pay extra to sit together but I do hope people start parking their kids all over the plane since that's what we all seem to want! It's tempting.)
Let's ignore special cases where you didn't have a chance to buy assigned seats, and focus on the vastly more common scenario where parents can easily pay to ensure seats of their choice.
Yes, it's nickel and diming by the airlines to make all seat assignments paid. And hating airlines is completely justified.
But I find the entitlement of parents, that other passengers should accommodate their parsimonious preferences, just amazing.
You can't have it both ways that you don't want a child next to you and just expect parents to spend extra money to accommodate you.
Easily pay? I assure you it isn't easy for a lot of us. The irony of your use of entitlement.
If you can't afford to fly then drive.