I think discussing crackers is taking it away from OP's intention. Mpje- just ignore the 'haters' - I totally understand your food choices.
With regards the nanny -where is she from - what is her cultural background? It depends entirely on family dynamics- whether she is live in and a 'part of the family' or merely hired help and whether the arrangement includes her food as well as board. If she eats the same meals as you then it doesn't make sense to create an 'us' and 'you' with snacks.
If you are worrying over expensive food being eaten - it is an odd one, you are rich enough to buy expensive food but not rich enough to be generous with it or share it? It make you look like you lack hospitality/generous spirit.
But I am sure you are not thinking of it in that way - I do believe you are genuinely thinking she doesn't enjoy eating this food. And I think that is the point- right?
Perhaps when you go food shopping with her you let her choose her own snacks/food - that way you get the message that there are some snacks for her and some for you. You can always blame DH !
We once had a lodger (child of family friend) and we made the mistake of telling him to help himself to anything . After one big mistake (eating some special chocolate biscuits DH was saving for himself) - we just made it clear that 'help yourself to everything - except this shelf - as this is DH's special 'indulgence shelf'. Problem was solved.
I personally think giving the nanny her own space for food and her own fridge is wonderfully generous - she may appreciate the freedom to put her own food in. You can always do it under the guise of not wanting the kids to eat certain kinds of food but realising she has every right to eat what she wants. But perhaps just ask her if she prefers.
Perhaps also ask yourselves what will DC think if she has her own fridge/cupboard space and how it will reflect on you as people. In the right circumstances - it is a kind act, in other circumstances it is unfriendly/inhospitable.