ex and new wifes effect on kids

Im at a loss as to where to go from here, appologies for the length i will make as short as possible.

My ex husband and I split over 4yrs ago due to his constant lying and multiple cheating. We have kids together. I have since found a new man who also has kids from previous relationship and we have just had a bub together, we are also married now.

I am able to get along with my husband's ex no problem we chat on drop off or pick up or if we see each other at a kids school/sport event and it makes everything so much easier. Now my ex and his new wife however are a different kettle of fish,he was a great dad before he met her and was very involved in the kids lives. Since meeting his new wife however he has slowly been distancing himself from the kids and has not attended school functions or sport events or anything significant and is also extremely unwilling to be flexible with having the kids. He no longer communicates with the kids except every second weekend when he has them,he used to call every second day. The kids are asking why he isnt around as much or why he is so late picking them up (can vary from 1hr to 4 hrs late). His new wife has never spoken to me except when he brought he over unannounced and i didnt like being caught off guard but was still polite. I thought as she had married him i would send her a msg via social media as i was not allowed her number to try and extend an olive branch, well apparently this was not a good idea. I recieved a scathing reply stating how i was an awful mother and how i am awful to her husband and i dont respect her and how my new baby is making me forget about my older 2. I have since blocked her and explained why to my ex, which resulted in numerous abusive phone calls and text msgs. He is taking her side and is making it very clear to the kids that I am awful which is hurting them and making them feel like they need to hide things from me which I hate. I have no idea what to do next or how to try and smooth it over, im trying to be the bigger person and let it go but when i am constantly belittled and accused of things i haven't done especially in front of my kids its really taking a toll. I have no idea why this woman hates me so much nor do I care really but when it involves my kids I cant just sit back anymore. My husband has held his tounge for the kids sake so far but when he sees how hurt the kids are and how the abuse effects me he is also finding it hard to continue to be neutral. Advice or experience with how best to go from her would be great as I am at a loss and want to fix it for my kids.