I don't want children in our college apartment
When did you decide that you didn't want children in your apartment?
I made up my mind last night
You shouldn't discriminate people based on their age. The renters at your apartment may be parents who need their kids to stay with them.
I can't focus on my work if there are children running around.
Even so, barring all kids from the apartment isn't right. You should come up with a compromise where there's no kids only during your study time.