Ever since my kids were about two years old they have had their own kids luggage. Before making our first purchase I did a lot of research into what is the best kid's hand luggage, and we inevitably started off with a Trunki ride-on suitcase.
Once their legs got too long for these, we progressed to a Trunki BoostApak, a backpack that doubles as a car booster seat and has now moved on to kid's luggage on wheels that double as backpacks.
Getting them their own kids' travel bags is a great way to get children involved with the packing process (although you may need to veto a few items if your kids are anything like mine, who think that ballet shoes and kitchen utensils are essential travel items!)
