Don't let his ex have access to you for starters. Block her from social media. There is no need for this person to be able to contact you via social media and in all seriousness, no need for you to be able to see what she says about you on social media. It's not helpful to anyone.
Agree to previous posters - be the bigger person. These are children. They are grieving the loss of the status quo - no matter how awful that might have been while they were still together.
I think you're going to have to ride it out for the time being. Or as someone else suggested, get yourself some counselling to help you deal with it.