I don't see a problem with unfriending them, you just need to bear in mind this may well be the end of any "contact" at all with your children's grandparents. It may feel satisfying now, but ultimately what has it achieved?
At what point is a woman allowed to say that she finds something so stressful, that she is not taking responsibility for it any more, when there are 3 people who are perfectly able to take that responsibility for themselves?
I felt some guilt - I could certainly add "keep up contact with ex's mum" to my todo list, along with keeping up the contact with my own parents (who do bother to visit), doing 98% of the childcare, working full time and maintaining my house (both day to day, and the fabric of the building), and having the tiniest sliver of social life (which also provides the safety net my ex doesn't bother to if something comes up and I need someone to look after the kids for the afternoon), sure, what's yet another thing on my already over-flowing plate.
I mean, it's just sensible, I'm the one that spends all the time with the kids and does everything for them, why would her own son know what they want for Christmas anyway?
Or I could say that an adult family member is responsible for their own relationship with my kids, and if I have to coerce it, then clearly they don't actually want it.