BigHammer: If they are on a learner licence, they must have a front seat passenger ("supervisor") who must have a full licence, at all times. No restrictions on other passengers in the back.
On a restricted licence, you must not carry passengers other than a spouse, parent, guardian or dependents, without a supervisor beside you supervising your driving.
This is a best answer.
I taught my wife to drive and we had a baby in the back for all the lessons. As far as I'm aware the baby had no issues with the arrangement and judging by the amount of sleep the baby found it quite relaxing. Although it was quite the opposite for me.