Minors don't need study permits if they are here with a parent who is on a work permit. You can simply take them to school and they will be admitted.
They will need a TRV (visitor visa), which will be easy to get since their parents both have approved work permits. Then, when you land, you ask the CBSA officer to give them a visitor record which matches your work permit.
If you forget to do this, they will be admitted for six months and you'll have to request an extension before the six months are up. It's easier to do it at the airport when you arrive, and this is a routine request that CBSA will essentially always grant (unless CBSA denies your entry for some reason).
That's all you need to do.