Does my sentence sound natural in wrtiting to native speakers? My sentence is "Parents believe that the kids in private schools have advantages over those in public schools.". By the way, I am making sentence using the new phrase "advantage over somebody".

Question

Answer

I would use singular: "the kids have an advantage over those in public schools".

If you use plural, it has a different meaning. It means there are opportunities available to them that are not available to the public school students.  In that case, I would write

"the kids have advantages that are not available to those in public schools"

Answer

Correct
another example

He has/enjoys an unfair advantage over us because of his wealth

Answer

Yes, that sentence could have been written by a native speaker.