When they are young, start by having them pick the toys they would like to bring. Get a case for each child so they have responsibility for their own things. Dollar or Pound stores are cheap place to pick up cases. This also minimizes the arguments between siblings. It gives them a designated space and when it is full that's all they can take. It will help them decide what they definitely want and the rest will stay home.
As they grow you could make a list of how many pairs of socks, shirts and underwear they will need. Then together you can work through the list and lay everything out on the bed. Then pack the suitcase together. If they do it with you, they will know where everything is. Then when you say go get your pajamas they will know where to look. It is a good exercise for them to learn to fold and take the necessities. Many of the airlines now have weight restrictions so minimizing is a good thing.
After you have gone through this experience with them a few times it's good to give them the list and have them place all the clothes out themselves. You can then check the items and approve of the selection or suggest others. The bottom line is at some point you are going to trust them to get on with it. If something is missing at the other end it will be a good teacher.
Children learn by example and doing. Be an encouragement to them as they are learning. Show them how to fold and roll items and to sort them so they can be located easily. The most important thing is not to leave it all to the last minute so you are all stressed. There is enough pressure on you to remember things like the passports, the food or camping gear. The sooner you can teach them the better off you will be. Then they can help you sort all the other things that need to be in place before you go.
Pack on and have a great time!
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