What Happened 2 My Ex??????

I'm gonna do my best to a long story really short. Ok, here's the deal. About 5 months ago I broke up w/ my bf of 5 1/2 yrs. It was really hard but I felt it was something I had to do. He wouldn't work or help out much & was always lying to me. Also, I had fallen for another man, of whom i'm in a relationship with now. So of course, in the past 5 months, me & my ex have had our good days when we can actually get along & be civil with one another, and our bad days when we just hate the mere thought of the other. My ex & I have 2 children together, 3 yr old twins(boy & a girl). There for a while he would pick them up in the mornin & keep them while I worked & then he would keep them every sunday night. But in the past 2-3 weeks, he's totally changed. I cant count how many times he said he would be there in the morn to pick them up & then he wouldnt show, call, nothing. He's called to talk to his kids on the phone once. He hasnt seen them in 2 weeks & i've recieved no word of him in almost a week now. I know he's been hanging out w/ his friends almost constantly, he's frequented the bar a few times, but still he cant find the time for his children. He dont work so I know he has the time. It's killing me cuz my kids ask for their daddy everyday n everyday I have to tell them they wont c him. My son is very attached to his daddy n he cries cuz he wants him, yet none of this seems to matter to my ex. He wasnt always like this. When we were together he was a descent father. I just dont understand y he's doing this to his children. I know he's still angry with me for leaving him but this is not the way to retalliate against me. I guess my question is what's your opinion? Why is he doing this to his kids & acting so very childish? If it helps, i'm 24 & he's 26. Thank u 4 your time n advice.