I fail to see the difference between exposure to your helper vs exposure to Cantonese, Mandarin, French, Hindi and countless other languages your child will get exposed to at school, friends house and at the playground.
Indeed.
Howard, there is a big difference between general ambient exposure to other languages and daily interaction between a carer and a child - even if the child-carer communication is in order to achieve a practical end it is likely to result in the child learning the language by repetition of what he hears. And I'd expect a good helper to make active efforts to encourage the child to speak/learn. On the other hand, unless I chose to send my son to a Mandarin playgroup (which I may well do), or he has a little friend who speak to him in (e.g.) Cantonese his exposure to other languages will be ambient at best.
Further, even more active exposure to other languages is unlikely to result in confusion with English; I have it on good authority (numerous bilingual families) that kids are very good at knowing which words belong to which language and don't mix them up mid-sentence very often. If he learns bits of those other languages it will be from native speakers and the grammar will be correct. That is not the same as learning a language from someone who does not speak it correctly (this is no slight on the intelligence of domestic helpers by the way, just a recognition that learners never get it right 100% - I'd have the same issue if my husband was the non-native speaker!). I hope that clarifies.