Everyone has provided great ideas for you to use. My husband was in the military and I was a teacher to military children (4 year olds), I was always trying to come up with different ways to help the children get through deployments- a lot of different ways have been told already.
1.) The good old paper chain- this is good when you have exact dates or guess-ta-mits. Kids like creating these and then take one down each day.
2.) Kisses/hugs from daddy- candy kisses/hugs in a jar (one for each child and one for yourself), have a picture of daddy "puckering up" on the inside of the lid. Then every day/night they get to give daddy a kiss and take a kiss.
3.) Calendar- when the date is unknown it is best to just have a weekly or monthly calendar. You can put stickers, x marks, or color the days in.
4.) Making care-packages- children love this! Get the free boxes from the Post Office and it is only $12 to send overseas (No weight limit). Have a different theme for each box. Also, when I was teaching and a student had a parent deployed, the other children liked to make and send cards to that student's parent.
I also had a few times in which my husband was away from our daughter due to his job. She was 3 when my husband was gone for 11 months, and she knew that it was going to be a long time before she got to see daddy. It was hard, but I always made sure she knew daddy loved her and we would see him soon. Also, it was a time for her and I to have special times- a special lunch, a walk, ice-cream, etc.
**Thank you to all the service men and women who defend our country's freedom**