Freeze your juice/milk boxes to use for keeping the other food cool. Make sure you pack dense foods in the bottom, softer foods in the top.
I make sandwiches ahead of time and I'm a Tupperware Diva, so they get packed in "hard" plastic. Tomatoes, and other "soggy bread" type additives get packed separately.
I don't do chips because they turn to crumbs so easily, even in my Tupperware, but crackers do ok. Sometimes I make "cracker sandwiches" with crackers and cream cheese or peanut butter.
A really good wrap is: large flour tortilla spread with peanut/almond butter or nutella, thin slices of apple or pear. Roll up, wrap in saran or lay in a Tupperware container. It slices into cute circles, too.
I also like flour tortilla with fat free cream cheese, sundried tomato pesto (a little goes a long way), turkey and sharp cheddar, spinach and thin sliced cucumber.
Carrots, apples, pears, oranges all travel well. I pack a lot of salads, just put the dressing in a small sealed container to use when you're ready to eat.
GORP (good old raisins and peanuts) with a little bit of chocolate chips or toasted coconut or yougurt covered peanuts or raisins, or craisins is great too. And gives those kids lots of sustainable energy.
I also like frozen grapes, even if they thaw they are yummy!
Family Fun has some home made granola bars that are yummy and actually pretty healthy, same with "trail mix". But my trail mix is just GORP with lots of other stuff thrown in, like sunflower seeds, coconut, banana/apple chips, dried fruit...
Don't forget hardboiled eggs, egg salad (I take the bread seperate with "salad fillings"), chicken salad, tuna salad....... for a fun treat you could take a bag of chips or crackers and use the "salad" as dip.
I prefer taking "finger foods" for car trips, and just make sure my wipes container is full for wiping off those little hands and faces (ok, my hands and face, too!). I am anti-disposable, and forks/spoons/plates can get gross in the heat. But, you could always take a (plastic) mayo jar with water, soap and vinegar in it to drop them in. (That's from my Mom, btw). (The vinegar makes sure any "ickies" die in the jar)
My mother, the queen of car-trips, taught me all of this. She also taught me to make sure you have a flashlight, gallon of water, small blanket, small first aid kit, sunscreen and a $10.00 bill Just In Case. (she kept the $10 tucked between maxi pads in her first aid kit-best hiding spot in the world!). I also keep a small bottle of Aleve or Tylenol (I have both), a $20 bill, a few quarters, Immodium,Benadryl, baby wipes, and a couple of plastic bags in my "emergency box".