>But you also can't just decide how much the child needs and then make the parent pay that much no matter how much they make.

This is basically how it works when you're married, though, at least in my family. Whether I'm unemployed or my work life is booming, the child cost the same for me as when I'm not. During bad times I liquidate my engineering tools / spend from savings / go into debt / sell my vehicle to take care of my kid if needed. On the flip side during the good times the extra money goes into investments and retirement. The amount I need to take care of the kid is fixed, with the amount I spend having virtually nothing to do with the amount I make. My level of personal real spending has changed very little since I got my first near minimum wage job after leaving home at 18; and definitely not linearly with my salary (at best I eat out more now, but that's because I'm busier making money). When the kid came they've always been a relatively fixed cost -- or at least unaffected by our salaries.

My kid would not be effected the slightest, better or worse, for my wage unless I was stuck below ~$12/hr for an extended time, so maybe it would make more sense to take a variable amount up to say $12/hr and then just a fixed price after that.

>doesn't mean that what you can afford is enough to raise the child well

Really depends. Some children grow up in situations where money is scarce but nonetheless have fulfilling childhoods that lead them towards success. It's also worth noting parents who can provide other things in lieu of money -- such as a homestead where they grow their own food and build their own house is perfectly acceptable in a marriage but somehow not acceptable as part of child support enforcement. It doesn't make sense.

>This has the benefit of allowing low earning parents to pay less

Again this is an odd choice. When I am making nothing, I still contribute half to the family costs. When I'm making 6 figures, my half stays the same. Both my wife and I have a deal where we pay half the costs. That way there is no resentment that someone is paying more just because they make more. It happens to be my wife makes significantly more money than me, but we still pay 50/50 costs of child and other bills with the remainder going into our own unshared accounts. It would be extraordinarily selfish and greedy of me for me to demand my wife to pay more simply because she makes more money, yet if we divorced this is precisely what the judge would order in many states. No matter how nasty the divorce I would simply return her share of her savings along with anything over half for spending of the child -- to not do so would make me a vile and greedy person who cowardly uses the violence of the state to unjustly take from others.

In our case the cost of the child is about 6-8% of our current salaries; so as you can see from child support calculators, about half of the money calculated by child support worksheets would be entirely for the lolz of the judge.

>So the compromise is to base the amount paid on the amount the parent makes

Unfortunately this isn't how it works, as above alluded. Once you make a certain amount it becomes your 'imputed income'. Once you set the precedent you can earn a maximum, the judge expects that to be your income you're capable of. You pay based on that higher amount whether you relax back to a lower paying job or not. In this way you're set up into a trap where if you try to get a higher paying job to save up to pay off years of child support, you're stuck in an even more fucked position and meanwhile if your kid is like mine and has a relatively fixed cost then the rest is bled off as backdoor alimony as a reward to your ex-spouse for divorcing you.