I am challenged as well with my 4 and 6 yr olds. I cook South Asian food mostly and they usually eat most that is served. However, feeding them protein is a task for me. Nothing besides store bought frozen breaded chicken nuggets or chicken strips works.
Getting them to eat potatoes and cheese, however, is easy. If its fries...I tend to bake them, with a drizzle of olive oil, as opposed to frying them. A good way to make potatoes tasty for the kids is to prepare potato cakes or patties with boiled, mashed potatoes, grated cheese (cottage or cheddar...or any other that would go well with potatoes) seasoning....and herbs of your choice. They may be optional, too. Cook the cakes/patties on a flat girdle, or shallow fry in a pan, till golden brown and crispy on either side. You can some other boiled veggies too..like beans, carrots or peas.
My kids love them with lots of ketchup!