My DH went away last year for 7 weeks and we told the girls where Daddy was going and looked on maps etc to show them where - travel agents are fantastic with great big world maps on their walls. The hugeness helped the girls understand why Daddy couldn't come home each day, why we couldn't visit him etc. Explained to them that we would talk to Daddy every day on skype etc.

They were pretty upset the day he left so we came straight home and made a chart to put on the fridge that showed all the days he was gone. On each day we wrote what we were doing, and I tried to fill the weekends with different, interesting stuff. Each morning they crossed another day off the chart and that really helped them.

I hope you can sort out some things for your dad while you're away.