With kids it is hard, sometimes they don't like to open up, or they cry and it's hard to understand their feelings because they don't know how to express themselves well. So being patient is the key, and don't get frustrated yourself.
Personally what I do is I talk to my kids usually about their dad, and we make pictures of daddy. We also write letters and I ask them to write down their feelings or draw how they are feeling. Then we usually follow it up with a scoop of ice cream. Then I tell them that, this is his job and we will be together one day soon and let's look forward to that day.