I often have a Premier Protein shake. I don't know how organic the ingredients are, but it is low sugar and high protein. I think my kids would drink them. You can find other brands of those I am sure.
Cliff bars, Cliff kid Z bars
We have been trying different things in the mornings while we drive. We tried Lenny & Larry's Fresh Baked Cookie. Those are pretty high in protein and relatively low in sugar. Kids get a "cookie" for breakfast on vacation. 2 out of 3 of my kids liked that.
We've also tried the organic breakfast biscuits paired with milk (you can get shelf stable milk). Not low carb, high protein, but pretty balanced and straightforward ingredients with some fat, protein, carbs