Would it be okay to use definite articles in front of the nouns?
It will change the meaning but it would be a valid sentence, yes. Just as the English sentence would change if you put definite articles in front of the nouns.
Ah, okay! Köszi! :D
I put Nem gyerekek tanulnak ebben az iskolában, hanem....Since I thought it was the children/parents and grandparents that were emphasized. .
In this school it's not children who study but parents and grandparents.