Actually on this one I don't know. Insurance can be quite a minefield - insurers can often choose not to pay out on policies for a variety of reasons.
As a nanny using MBs car for work purposes I am fully insured for this.
Do you have your own insurance, or are you a named driver on your MBs? Does it state on the policy that it covers you for children for whom you are paid to provide childcare? Some policies will phrase it such as ?Social, Domestic & Pleasure purposes and Business use in connection with childcare activities? (such as Morton Michel: MotorCare) but even with that I do not feel we can be 100% sure that in the event of a major claim the insurer wouldn't try to quibble out of paying. All we can do is hope to be able to prove that we took as much insurance cover as was available to us.
Am I insured to carry another nanny and their charge when both sets of parents have knowledge of this?
If that nanny, or their parents are paying you in any form... then I would say that you are not covered. However you are probably not being paid by them.
A general car insurance policy will say something like: any passenger travelling in or getting into or out of Your Car.
So I read that as meaning that as long as the person is a non-fee paying passenger, then you are covered.
It all really depends on the exact policy wording of your insurance policy.
Working out what is and isn't covered is often very very tricky with policies and thus all you can do is hope that the cover is sufficient in the event that the worst happens.