Is there ever a good time to tell your child the truth about their dad?

I have 3 children, 9, 6, 3. The first two are from my first marriage that ended when they were 4 and 1.My ex is an irresponsible spoiled golden child whose family makes excuses for him at every turn. So many times he has no call-no showed these kids. I have told him that he can have them WHENEVER HE WANTS because I know they want to see him. He takes them on Tuesdays for about 3.5 hours and only because he takes them to his grandparents- who are the ones who set up Tuesday meetings because they want to be able to see their great grandkids.On January 1st (Tuesday), he was supposed to pick the kids up at 4. He never showed. We figure whatever, my husband was playing with Legos with all the kids and it was nice they didn't have to stop. At 5:15 the phone rings and he apologizes for not showing because he partied so hard on New Years Eve that he JUST WOKE UP.At my daughter's Christmas concert, me, my husband and my parents were in the very first row. At the end of a song, she looks at me and asks, "Is Dad here?" It broke my heart because I had to say "I don't know." I whipped my phone out to text him in case I just hadn't seen him. And even though he told her he would be there... he wasn't.He took them in the afternoon on Christmas (Tuesday) and hadn't even gotten them a single gift. He is a CO and could have gotten them just one thing. I know gifts aren't everything, but they are still small and he should have gotten them something.Despite his selfishness, the kids love him and are always so excited to see him. I would need more fingers and toes to count the times he has made up some stupid excuse for not showing.I asked my 9 year old, "Does it bother you when your dad doesn't show up when he is supposed to?" and she said no. I asked why and she said, "Because I miss him and I want to see him."I am always civil to him around the kids and don't say exactly how I feel about him and his actions around the kids, even though I want to punch him in the face every time I see him for putting these kids on back burner and never making them a priority.Is there ever a good time or good way to explain to my kids the way their father is? They have a stepdad who dotes on them and loves them and is by my side as a parent every single day. They see their father 4 days out of a month- usually less.Is it wrong for me to want to explain to- at least my oldest- that her father doesn't show up because of his own poor, selfish choices? via /r/Parenting http://bit.ly/2QipRRW