It does not matter if they are US citizens. I think it totally depends on the school if they will allow them. I know of visitors trying to enroll their kids in school and being denied. I also know of somebody having a kid visit on a visit visa and enrolling him in school without a problem.
Here:
http://www.cic.gc.ca/english/study/study-minors.asp you will find information on study permits for minor children. It says that minor children of temporary workers or students don't need a study permit, it talks about children not accompanied by parents having a custodian. However, it does not talk about the possibility of visitors wanting to enroll their kids in school.