They could just do what we do with every other crime, and only punish people AFTER we find that they're guilty. You say this like the parents absolutely NEEDED to be supervised, despite saying "we only did this because we didn't realize it was illegal" - this is not exactly a High Threat situation.

If CPS is going to force this sort of thing on parents, they could also reasonably be expected to compensate for the involved expenses - force CPS to pay for a nanny if CPS really thinks that's essential. If CPS can't afford to do that, then we as a society have already voted that the nanny wasn't actually worth the cost.