All parents have the responsibility (just as a matter of natural law) of making sure that their children are raised and educated in a healthy way, so that they can become healthy adults. Catholic parents have the additional responsibility of making sure that their children are educated and trained in the Catholic faith and in Catholic morality.
Now obviously, bad stuff can happen to a parent, like dying. It obviously isn’t a sin to have young kids that need you, and then happen to get hit by a car. A parent can prepare for this situation by having a backup plan for how to get the kids raised, often by choosing godparents or responsible family members ahead of time. The same thing is true if a parent should become ill and be physically unable to care for the kids.
It is also possible to send kids to stay at a boarding school or a sleep-away camp, for weeks or months or even years. In the old days, it was fairly common to send a few kids to stay at a relative’s house or a friend’s house over the summer – or even for years if the parents were poor, or the relative/friend had better schooling or jobs available for the kids. In the Bible, Samuel’s mother and father sent him to stay with the priest Eli at a very young age, so that he could serve the Lord as he had been born to do.
This is not neglect or abandonment; it is delegation. So obviously one can delegate parental duties to other adults.
If one can delegate parental duties, then for a sufficiently serious reason, one can permanently delegate them through adoption, and even through closed adoptions. And that is not a sin.
Adopting kids out for a frivolous reason would probably be a sin, although it could also be a sort of cry for help from a parent with psychological issues. (And in that case, it would be more a symptom or a breakdown than a sin.)
I hope this helps!