My father-in-law is a type 1 juvenile diabetics and prefers trail mix to munch on. He also enjoys popcorn and unsweet tea. He has done plain or honey roasted peanuts from time to time.
We don't eat low carb, but I always keep almonds in my car to snack on. I'd bring a loaf of bread to go with the things you've listed (just to give the kids something that would have some carbs).
How about a lettuce rollup with meat and cheese in it? Take a large leaf of lettuce, thin slices or meat, cheese, roll it up, pop some toothpicks in and slice it up? Add whatever condiment you'd like for "glue" as well. They also have low-carb tortillas and you can do the same thing. For your teenagers, take dollar buns and make small sandwiches without the condiments in advance then you can have something heartier for them.
You can't go wrong with pinterest either. We also do a lot of make your own trail mix PRIOR to leaving on road trips. Then it's customized for the person. Everyone gets their own snack box and they fill it with their favorites. Minus items that need to be cooled. Then I try to separate them into gallon ziplock bags or containers (depending on my cooler I'm using) labeled.
In honesty we take a cooler that sits in the floorboard in the back with my daughters. They use it as a table for the laptop so they can watch movies. We usually make some PB&J sandwiches, muffins, or ham and cheese crackers.