I feel it is unusual for a nanny to bring their own food to work. Due to the nature of the job, the nanny is usually involved in cooking meals for the children, so it makes sense to cook additional quantity so they can eat with the children.
Working for longer than 6 hours a day, an employee is entitled to a break of 20 minutes (see here). However nannies can't really have a break, as they can't leave children unattended. So there is a bit of give-and-take involved with regard to rest breaks, such as nanny having quieter moments during the day - such as to drink a cup of tea.
All my nanny employers and one employer in a different line of work, have provided tea/coffee for free. While it may be considered a perk, it's things like that which keep employees happy. If employees start providing their own, then they argue over whose is whose - not a problem in nannying, as you only usually have one nanny... but in other work environments it does become an issue, similarly whose going to wash up the mugs, whose stunk the staff room out with their microwave meal!
Nannying is different to working in a larger organisation, it's much more of a personal service. That I feel means it should come with some personal perks, as a way of compensating for the more negative sides of the job - such as long hours without a break, stinky nappies, vomiting children.
As a nanny I would not be happy if my employer didn't provide tea/coffee/lunch. All meals whilst on duty I feel is pretty normal - I occasionally have some toast for 2nd breakfast (having had breakfast at home at the crack of dawn). Same goes for afternoon tea - sometimes I will have some, sometimes I won't. Lunch however is always something I have and it is generally the same as I am trying to get the children to eat.
I get a weekly kitty for misc expenses. That I use for various things like paying for toddler groups, admission charges for museums/activity centres, swimming (I deduct a contribution towards the monthly membership I pay for the child), plus odds and ends - be they paper/pens or food related.
I deliberately do not track the expenditure as I know that if I add up all the things I pay for over the week, then it will exceed the budget. That's how I work though... some weeks we can spend a lot, others we can spend little. Keeping the kitty the same each week, with luck evens out over the year.
If we go swimming, I mentally deduct the contribution to the monthly membership. We then often have something to eat afterwards... I pay for that. I may make a mental note of how much the child's meal cost but don't tend to add on what I had.
Why don't I add that meal? Well, we could have gone home and had something. I choose to have it out. It's like when on outings... you can take a flask and sandwiches, or buy things out... or more often we tend to do combination of both - coffee much nicer if made just before consuming, rather than in a flask.
So it's all about give-and-take I feel. You as the employer need to be offering a desirable work environment, which includes food/drink whilst on the premises. Once off the premises it's more difficult, as could come home for lunch then back out again (but that would mean mileage claim goes up hugely).
Getting a bit long... not sure this answers your question or not. Does it help?