The problem with eugenics is proposed solutions, or criteria based of prejudice.

I claim it’s both fine and correct to state that the wrong people have too many children. You’re the wrong people if you have more children than you can adequately care for, to raise sufficiently for them to have a successful life. You’re the wrong people if you have children you’re not prepared for or otherwise can’t commit to raising or don’t have the ability to raise.

It’s wrong, eugenics, when

• it’s prejudicial such as based on race, culture, religion
• you’re judging another person’s worth, their rights on that worth, or their opportunities on their worth
• you take it to an extreme, such as only the wealthy should have children.
• you prescribe a solution that imposes your will on other people, or worse, legal or medical intervention

Better answers include

• better education helps people make better choices
• better medical care helps people know their choice will succeed
• better safety nets help each child succeed even when their parents made a poor choice or had unexpected life events
• better childcare options help give the parents a chance to succeed with trying to earn a living while raising a child