The child seatbelt issue is a good point, and I think ultimately though it comes down to whether or not an injured party ends arising.

The main issue with the myriad of ridiculous laws that cops enforce is that it gives the state the power to decide certain things you may or may not do, under the guise of public safety and preventive measures, even if the only person you may be harming is yourself, as you stated with the seatbelt thing. And because of this beauracracy what usually ends up happening is nonviolent people are apprehended in very violent ways, and are now deemed criminal by the state because of preemptive policing that may or may not actually prevent a crime. Not to mention the ridiculousness of making us pay with taxes to house nonviolent people in prison, if they end up there.

I think things like public urination or defecation could still easily be addressed in a free society with strict private property rights as a violation of private property. Ultimately though it would be up to the property owner as to how they'd choose to deal with that.

Just some food for thought!