I agree there is a lot of negativity about language learning among people who are not familiar with it. I don't know why this should be because it actually has huge benefits
I don't see that on this thread, the most insightful posts have been from people who speak more than one language. Although, very few have come on and said, yes, I hired a nanny speaking German for a year or two and now the children are fluent. In fact, none I don't think, so far.
The OP specifically says she doesn't just want a numbers/colours level, in which case the commitment is high.
I also think people underestimate how difficult it is to get toddlers to speak a language if they don't want to and no-one else in the family does and they know you understand English. I know heaps of families where it all starts off nicely saying hello in three languages, but then the child clocks you get English and ends up not replying except in English. I have also known children who get very distressed about not be able to communicate- and some children who enter an immersion environment such as English school find it very hard going and quite distressing, though most do incredibly well once you get going. Three or four year olds can sometimes take to things very easily, other times, very stubbornly. If you have difficulty getting them to wear a coat, doesn't it make sense that sometimes they are not 100% compliant, or have a big old paddy because they can't reply to be understood.
What I'm saying is that it is harder work than the OP's initial post seems to suggest, her goals are slightly unrealistic, and the key point is her husband isn't on board, so won't necessarily provide the support if things start to be difficult or the toddler doesn't want to speak German/only speaks English. The families I know who have made bilingualism work either a) have the whole family speak the home language, English outside or b) do OPOL like a religion and spend a lot of time in the country concerned/have school reinforcement/do language lessons (e.g. in Arabic where there's a different letter system to learn).
Everyone is right, there's everything to gain by learning a few words in another language and getting confident in speaking them. I wouldn't discourage this at all, but the OP isn't realistic and her husband isn't on board, and it would be a shame for this to be a stressful placement for all concerned because of this, especially as the nanny may not want to speak German constantly whilst managing two small children in the UK (and what happens when she goes out and the cat is out of the bag when they hear her speak great English!)