Backwards. The government knows who has kids because they do things like issue birth certificates and operate state schools. In a lot of places it's legally very difficult not to be listed as a parent on government records.

So why should I have to prove I don't have kids when the government can know I'm not on any of those lists?

> How do you propose they find out, without asking you to prove it?

How do I propose the government know if I have kids? I'm pretty sure they already know that I don't?