Did anyone also nanny while they taught? Would that go against the visa rules?

I did not nanny. It would go against the rules.

I disagree with Justine. Many of the teaching assistants nanny, babysit, or tutor to supplement their income. As long as it does not prevent you from doing your job—ie. teaching as an English language assistant in a French school—you can certainly do so. Au pair jobs are not permitted, as this would interfere.