I have this same problem! Protein, and lots of it, is the answer. I have about a 20 minute drive tafter I pick my kids up, so it is crucial that I bring them something in the car to eat. I have started bringing "chocolate milk" which is really Carnation Instant Breakfast drink. between the powder and the milk, they are getting a good dose of protein. (Note: I am going to look into switching this to a vanilla flavored protein powder and carob powder mix - I don't think they need all the additives that are in the Carnation mix).
So the milk gets us home, but won't last us until dinner. Thus, I invented "Mama Tea" when my kids were just starting school. I had always had a British style tea and snack for myself around 4pm, so I expanded this to be a mini meal for all of us - just like the Brits. The other benefit is that it provides a civilized, focused moment for us all to sit down at the table and share news about our day. When they were younger, I would put out a multi-compartment dish filled with nuts, fruit, cheese, etc. anything that was nutritious and had protein. I would serve them decaf English breakfast tea with lots of milk and some Agave sweetner (although my youngest has now gravitated to decaf chai, with milk) and after all of the "brain food" was finished, we could each have a cookie.
Now that they are older, the buffet style of little finger foods is no longer substantial enough for them, so now we still have "Mama Tea" but with things like quessadillas, mini hamburgers (the precooked kind you just pop in the micro), chicken flautas, or chicken nuggets. All but the quesadillas are things I buy at Costco; I'm not psyched because they are prepared food with additives, but they are eaten in small quantities and get some points for having lots of protein (and being easy!)
By now, everyone who knows us knows that we have "Mama Tea" around 4pm EVERY day, and it is just as regular as breakfast or dinner. The key is to think of this "snack" as a fourth meal, and to take it just as seriously as you would dinner - they really need that much food when they are in school!
Oh, one more thing I try to do - when packing their lunches, try to put some extra in so that they can eat it after school if necessary. It is not a solution, but it will buy you some time to get the real food in them!
Hope that helps!