have you thought about applying the migration yourself? as your kids stay in Canada to study. since your kids are canadian, and you might have the change to apply the for the PR to stay behind to take care of them. check the canada official website for more details
No need to migrate as my neigbour has similar case. She has 2 kids and she came with special visa, but I dont know what exactly call.
She can accompy her kids as they were only 9 and 11yrs. It is a big money as both kids have to pay overseas student rate, rent a house and all the living expense. Try to search the web