Sorry MHL you are going through this. What comes to mind is you are trying to fix this for your children. Well, it can't be fixed. I am not sure why you are bothering and getting angry over something you can't control.
I get and understand that you want your ex to be a responsable parent, but she is unable to do that today. For whatever reason. I have been down that road and I get the anger and the no answering and responding to calls, texts from the kids. Being on the other side now, it was the depression battle that my h was dealing with that kept him as an irresponsible parent. It didn't mean he didn't care, but he was battling a deep deep depression that prevented him from being available to our kids and to me as well which he wasn't strong enough to get himself out of.
I can only suggest that you do the best that you can with your parenting which sounds like you are doing awesome at and let the chips fall where they may in regards to your ex. I am not so sure I would encourage a relationship with your kids and ex since they seem to be so hurt by her non-responses.
When my h was not home and the kids would call and he would not respond and they were hurt. I would simply say something like I am sorry dad is not returning your call. You have left him a message, so I am sure he will call back when he can and then just left it at that. I just let my kids form their own opinion of the situation. This way my thoughts and feelings did not dictate their relationship.
Best of luck. I wish it were a different way for you, but I guess this is life as we know it today. Doesn't mean that couldn't change though in the future.
Me 50 H 42 S 22 S 9 D 7 M 12 T 17 H moved out 8/2006 H moved home 1/2007 for 3 weeks H moved home 5/2011 for good
"Learn from yesterday ~ Live for today ~ And hope for tomorrow"