When Dads talk about maintaining their relationship with their children and then don't live up to their responsibilities, it is very sad. Here are some suggestions:
Do you have communication with your ex to discuss things from the children's perspective? He can't change his behavior until he clearly understands the disappointment and rejection his children are feeling. If you can discuss this without judgment or making him defensive, perhaps he'll get the "message."
If not, it's best to talk to your children about not having expectations regarding when and how they see Dad. Tell them everyone does the best they can with what they understand about life. Perhaps in the future Dad will understand more about how disappointed they are. In the mean time, they can decide whether to see him or not -- and how much emotional involvement to make in their relationship with him.
There is always hope for the future but you certainly want to protect your children emotionally from being hurt in the present.
Hope there is so value in this for you.
Best wishes, Rosalind Sedacca, CCT\
The Voice of Child-Centered Divorce
http://www.childcentereddivorce.com