Do they like PB? How about a wrap with PB & banana. Easy to make, easy to eat on the run and full of goodness !
That and a glass (bottle, carton) of milk would defnitely carry them through. Depending on how old, perhaps only a half a wrap so they don't get "bogged down" by the food in their stomach ?
Sandwiches are also a good idea with carbs and protein. Fruit is quick energy also and in combo with a protein you have it made.