I will second what hummingbird had to say about the resiliency of kids.
You will also notice a change in their behaviors and their acceptance once the move is over. It may be a roller coaster of feelings but it sounds like you are doing a great job talking with you children. That is very important especially during a time when you have sooo many things to get in order.
Looks like you are just about to make the big hop over, congrats!!!
Promises like people will write from home or will phone often are easily made, but that's often times completely out of our control. Help your children focus on activities that they can control. Perhaps consider giving them simple assignments at the aiport (look for a bathroom, water fountain, gate number etc), on the flight (collecting trash to hand the attendant, watching for the bathroom light to go off, etc) or upon arrival (get a trolley, count the bags). Make it fun instead of heavy responsibility.
Another idea is giving your daughters small journals/scraps of paper/envelopes/whatever, to write their experiences, draw pictures, share feelings, collect ticket stubs etc. This is an activity that can also take place after you arrive and start to get settled. It can be an activity like creating a simple story book for other kids who have to go through this. They can write things like where they are going, how they are feeling, what's the same, what's different, what they miss etc. This could become a treasured moment when they get older or something to share with the children at their new school. Your daughters are doing something many children will never experience and having something tangible is ideal for their developmental age.
We're sure you are doing a fabulous parenting job! All you can do now is jump in with two feet. Safe travels!!!
