Plenty of things to do in their hand luggage, book, comics/magazines/puzzle books/drawing pad and coloured pencils, etc. They can watch the film inflight too. Electronic games may not be permitted to be played in some cases if they don't have a flightsafe mode, but could be played with in the airport and at the destination.
Chewy sweets, mint or fruit types for take-off and landing to help clear their ears. Or you could get 'earplanes' ear plugs which have valves in them to release the pressure gradually.
Plain still drinks or water for the flight.
Check in, then go through security straight after. You can then browse the shops airside for entertainment, sweets and drinks, and any food you need for the flight. This helps take up a bit of time, but you don't want to be rushing at the last minute to get through security and find your departure gate, this won't help tempers or anxiety.
I think it helps to go through the various procedures with the DC beforehand so that they have some idea of what to expect, and also why the procedures are the way they are. Do check their packing especially for sharp objects, and liquids/gels/pastes which will be confiscated unless in the regulation clear plastic bag. They are still very strict about the security rules. Hence buying drinks and stuff airside.
I think you can see plans of the interior of the airport public areas, and see what shops and facilities there are. I think this will help you too, as you will have some idea of what to expect if the airport you're flying from is unfamiliar to you. Also keeping occupied will help the time pass more quickly and less time to worry.
Have a lovely holiday!