Makes sure your nanny knows your views regarding the food the children eat. While sausage rolls and coke can be good at parties, they are not really suitable in my opinion on a daily basis.
Bakery - nothing wrong with that, though far better to bake at home... it's not rocket science, and children love to help.
I recommend the following books:
The Dinner Lady - Jeanette Orrey ISBN:0-593-05429-6
The River Cottage Family Cookbook - Hugh Fearnley-Whittingstall ISBN: 0-340-82636-3
I also like a book of Nostalgic recipes from the 1940's and early 50's.
Feeding The Nation - Marguerite Patten OBE ISBN: 0-600-61472-7
Pick some of the easier recipies from these and ask nanny to follow them.
A simple one to start with is Pasta, Peas and Bacon (from The Dinner Lady), this really is easy and children eat it! Then get more adventurous and try Toad in the Hole - it is simple, but getting yorkshire pudding batter to rise is the challenge.
Cakes are easy to make... start with Muffins, as once mastered, you can try different flavourings: choc chip, cherry, apple, banana, blueberry to name but a few.
Corkflake Crunchies (cornflake cakes) and Krispie Cakes are also very easy to do, plus you can experiement with different types of ceral. Wheatabix sort of works!
Flapjacks are also easy, though I do still tend to overcook (burn) mine, so watch the timing!
Once those are mastered, your nanny can try making bread. I found a burger bun recipe on the web (from the US) that I found easy to make, alas can't locate a link for it right now. Bread rolls I find easier than loafs, so perhaps start with rolls. Also flat breads work quite well - see River Cottage Family Cookbook for flatbread recipe.
Plus, I find it's great to know how to:
Cook Rice
Make a basic white sauce
Rice, Sauce, Fish and Peas/Sweatcorn I find goes well - the children mix it all up into a gooey mess. It's lovely though, even works with tinned fish, though I find smoked fish is better.