Depends.
Is it snacks against boredom, snacks because you want to absolutely minimise stopping times to loo breaks only, or snacks to stop travel sickness?
If it’s the first then you could consider packing them each a little bag full of mini mysteries - tiny notebook and pencil, sticker sheet, an optical scavenger hunt (tick list of things to spot - orange lorry, green car, 3 motorbikes in a row, cows in a field, that sort of thing), set of safe challenges to do from a car seat - wiggle your big toe without the others moving, tense and relax legs and arms, breathe in count to give breathe out, count your teeth with your tongue, that sort of thing mixed in with the occasional boiled sweet, and let them open a new one at any set interval.
If it’s portable meals to reduce stopping times then sandwiches with non-spilly fillings. So ham, peanut butter, honey, cheese spread, rather than grated cheese or prawns etc. And just standard picnic food but of the less crumby variety. Commercial sliced bread not baguettes or pasties, sucky yoghurts not ones in a pot, apples (pre-slice and reassemble into apple shape to reduce browning) and carrots and sugar snaps rather than leafy salads and squashy berries. Biscuits rather than cake.
If it’s snacking against carsickness, ready salted crisps, ginger snaps, rice cakes, boiled sweets.