They have US citizenship by virtue of them being born there. See
here.
I am not sure if the Canadian citizenship will be automatic or if the parents have to apply for it, but it would be granted without any issue.
So, they will be dual citizens, which is pretty cool.
EDIT: I just read a little further in the link I gave. On p. 46 it has the rules for Canada, which states that the'll have Canadian citizenship immediately too. If they don't live in Canada and apply for citizenship, it will be revoked at age 28. Presumably, the parents will apply for this on their behalf immediately. This will give them permanent dual citizenship unless they renounce either one.