IMO yes she should be cooking.
Get her a recipe book (look at student cook books - they're often very good at giving simple, healthy recipes) and do a weekly meal plan for her at first.
Pasta, tomato sauce from a jar, tuna from a tin, cut up some carrot and cucumber sticks = simple pasta bake with crudites
Chop up some vegggies, steam them, put a couple of chicken breasts in the oven, boil some rice = simple balanced meal
Chop up some veggies, shove all in oven, boil some pasta = oven roasted vegetables with pasta
Brown some beef, chop some onion, carrot, potato, add a tin of tomatoes and slow cook = casserole
Frozen breaded fish and frozen chips in oven, frozen peas + sweetcorn in a saucepan = fish and chips
Cooking is not rocket science and very few people expect cordon bleu standards from a nanny. Ideally preparation should be under 15mins and it shouldn't need too much looking after while it cooks. Not all food has to be from scratch every day too.
Does she know what's nutritionally appropriate for your DCs?
I think this is often one of the problems with ex-nursery nannies. They can be fab in many, many respects - lots of good craft activities, experience with lots of different children - but they rarely know how to cook/clean and aren't used to having to knuckle down and learn something because in a nursery there's often someone in your team who can fill in for you. She'll soon learn!