First of all, hope you get some good news on your dad when you get there!

My DH had to go to NZ a few weeks back for his sick dad. We have a 4 yr old and a 2.5 yr old. We told them both the same..

Grandad is in hospital and is really sick. Daddy is going to have to go on a big plane to be with him and won't be home for a long while (otherwise they would've thought tomorrow). But you can call Daddy whenever you like, and Mummy will be here..

My 2.5yr old started getting a bit upset after 2 days when he couldn't find DH anywhere, even checking the bed (DH works shifts and often sleeping in the day. DS is happy to see him and then leave again).. We called twice a day on skype so they could talk and I had no issues answering the question 'where is Daddy' every time he asked, and it was heaps, LOL..