Also, think about the airport you go through. It may be expensive, but may also get the DCs invested in actually eating stuff - if they get to choose a snack in the airport after security. And it may use up some boring waiting around time too.
In Dublin airport, for example, there is a very good place that does made up smoked salmon on brown bread, or prawn salad, fresh fruit salad, and also makes up sandwiches with whatever you want in them (lots of choices).
Most UK airports I've been through have something like a Starbucks or Pret, so it will be made up but there may be something the DCs would happily eat there (that you buy as takeout).
If you are going to the US (or indeed anywhere outside EU), check the customs rules. Some places don't allow fruit, others don't allow meat, others dairy etc. So I had to leave an apple in pre-customs clearance in Dublin for a US flt, but going to Canada through UK was able to bring fruit and cheese (to eat on board).
What sort of things do they like, and how can they be made transportable. Make sure you have drinks - plenty of plain water but also a treat (there will be drinks on board too) like a J2O or something fizzy (DD much prefers juice as they will have fizzy drinks to give from the trolley). We usually bring chopped up apples, box of raisins (the snack boxes or some leftover McDs kids meals ones
), we get snack packs of cookies or oreos or (I might get some Lu biscuits in Brussels), M&S do mini pretzels and cheddars and snack-sized cheeses (and tiny hummus pots too - but they are garlicky!).
You could always have things in a hard plastic lunchbox so they will get less bashed. You could even do a full on picnic - sliced ham, their favourite cheese, cooked prawns, salad, cherry tomatoes, chunks of pepper, breadsticks, pasta/rice salad etc.
Airline food is not that bad though. We mostly do picnics now for shorter haul flights (where we have to buy food on board) and usually its a case of buy NICE food in the airport rather than same priced but less nice stuff in the air.
A tray of airline dinner on long haul is usually salad, bread roll and butter, main course, water/juice, tea/coffee, a drink from trolley, and a desert (brownie, chocolate mousse, cheesecake, and icecream are ones I remember in recent years). Often there is a piece of cheese and 2 crackers as well. If it is transatlantic, you will probably get a snack too before landing - slice of pizza is what US airlines (various not just "US Airlines") often give you, or a sambo from BA. Breakfast is often juice, muffin, yoghurt and a piece of fruit, tea/coffee. So there will be something edible in that lot - and you will find food at the airport on the far side if all offerings are refused.