Saying goodbye to Daddy for my kids is extremely hard, especially because he is involved in so many of their activities and spends a great deal of time with them.
One of the activities we have always done is set up Skype dates. This was a set time (if the internet was acting right over there) that the kids could get on, show him projects they created in school, sing, dance and just act silly. We also made great use of the webcam for the holidays so that daddy could watch them Christmas morning.
Also encourage your child to write their parent letters or draw pictures that can be sent via snail mail. This will make them feel special and also encourage creativity.