Please don't be offended by what I'm about to say. I'm trying to be objective. As crappy as her maternal instinct is, she still is their mother and the kids love her despite this and she should be participatory in their lives. IT IS A SHAME that you have to guide her like a child but it is what it is and that's why I'm sure you have custody. I'm sure in a moment, they would never want to live with her but they must want to see her if they were using the calendar to remind her of visitations. I read that and it made me sad for the children, acting more like an adult than her.
I think it makes you look bad, in the very least, the childrens' eyes when on one hand, mom is bitchin' to the kids that
DH is keeping her from them and then you turn around and take their 'mommy' calendar away from them??? I think that's reinforcing what she's telling them.
What harm is it that she sees them one weekend a month? Is she abusing them? Is she endangering them? I think the children are being set up for disappointment and on some level, you have to take responsibility for that. What is the harm in reminding her? As pathetic as it is for her as a parent, I think it's truly an example that the children are the victim here.
Give the kids back the calendar and let them deal with their mother's unreliability. They already know her for what she is.
You know mom isn't going to get her old visitation back when you can prove to the judge she hasn't shown up in 3 months, she's gotten letters from you addressing this and she doesn't show unless you remind her. Also, it's going to take some money to make this happen on her part and do you really think that'll be the case? And if so, you have the proof in your corner.