Hello. I was wondering if I could receive some kind of help please.

Hello. I was wondering if I could receive some kind of help please. The father of my kids and I got joint custody in January, and were ordered to communicate through parenting app. He hardly ever uses it and waits till he is picking up or dropping off the kids to belittle and harass me. My 4 and 6 year also come to me every other week and tell me that their grandmother, his mom, is constantly talking bad about me to them, trying to convince them that I am a bad mommy. This is emotionally and mentally draining. I dont care what the grandmother says but dont say it to a 4 and 6 year old. I am desperate for help because I am not a bad mother but the father and his mothers constant harassment and accusations are draining. Anything would help and ben appreicated. Thank you.