I don’t know about the UK , but High School language classes in the US , unless it is an IB or AP class, are pretty basic. Students normally never progress beyond being able to make simple sentences and, even in simple sentences, most of them have poor grammar and especially poor pronunciation.
I am aware that Canadian French pronunciation is very different from Parisian French , but the standard written language , except for a few differences in vocabulary, is the same throughout the French-soeaking world. I am obviously assuming that the nanny would speak standard French to the Sussex baby, albeit with a Canadian accent, rather than a dialectal form of Canadian French with nonstandard grammar. I would favor a French Canadian nanny over an European French one because, even if she has a “ funny accent”, her English pronunciation on the other hand is likely to be much better, especially if she is from Ontario, but from the local French school system.