It's not just about fines, many countries have support for families with children. I don't think the rich should get more money per child. For fines, it absolutely makes sense.

A harsher alternative is to stop using fines altogether and instead give "prison micro-sentences" - a few hours or days in prison. It makes perfect sense - when you pay a fine, you lost a bit of your life by working and not having anything to show for it at the end. So why not make it direct and just take a bit of time directly from the person. It nicely sidesteps various tricks how the rich hide their assets, too.

Of course, the administrative overhead would be much larger, but then the offenders could take some part in maintaining the prison. Nothing would be more humbling to a privileged person than cleaning the prison toilet.