I'm sure my situation is common but I'd really like to hear what people really think. Here is a little bit of history to explain my situation:

My ex-husband decided he only wanted our children one weekend a month and half of the summer. He has had very rare contact with the kids except for when I had to pay him to watch them for almost 9 months while i worked that was almost 5 months ago. He called them 5 or 6 times over the summer but has now decided that I have to make them call him every night before they go to bed because he's busy. My children are 8 and 9 years old. I have told them that they can call their dad whenever they want on several different occasions and I have sent him my work schedule so he knows when I am home for him to call them. They have rarely asked to call him.
Do you think it is my responsibility to go out of my way to make them to call their father every night?
At what age do you think kids should be allowed a choice in regards to how often they have contact with their absentee parent?