Perhaps "controller" and "controllee"?
"Boss" and "underling" conveys better the meaning.
Anyway, I find the term "parent" not adequate for those who had abusive parents, have lost them young, or have been abandoned. It's also painful for those who can't have children and thereby can't become parents. Moreover a parent\child relationship has already a clear and different meaning, especially in OOP.
"Worker" may not be good for the far-right, always fearful that workers of the world could unite, size the production servers and create a cloudkhoz. And a worker, unlike a slave, has to be paid and may want a pension when it's garbage collected.
"Leader" and "follower" has also bad connotations - who want to be a "follower"? Do you imply my code lacks "leadership"?
"Primary" and "replica" have a different meaning than master/slave. "Replica" is usually passive only. Even "primary" and "secondary" have a different meaning - it's a less strong relationship.
Maybe "Lord" and "Vassal"? Just, vassals were not always reliable.
But maybe, for Python, "Scrooge McDuck" and "Donald Duck"- with duck typing, it looks good.
Albeit, I have issue with the Python name itself, some people are really afraid of reptiles. If it quacks, it's a duck.
The good thing is that with each language and framework using different terms, people will understand even less how to write such kind of code, and quality will become even worse.
But the Newspeak attempts will have reached their goal to hide actual, real slavery.