Travelling on a plane? Bring a double stroller...Kids don't like walking much and depending where you are going they can sit and relax. Doesn't make them as tired and grumpy either lol.
If you have a portable small tablet for them too watch cartoons or play games that will be a life saver as well.
I did a 17 hour train trip in october with my now 3 year old and 19 month old daughter. Like kelx mentioned I had a tablet with Peppa Pig, Paw Patrol and other cartoons the liked with games and I brought a few of their favorite books and crayons a coloring book and a mini play dough pack I picked up at the dollar store. You will know the best entertainment for your kids. If your kids are fussy eatters like mine you may want to bring their favorite snacks as well. If travelling long distance bring pillows and blankets.I hope you and your family enjoy your trip.