That suxx JB... I know for a few years, maybe three and four years ago, my W's XH would routinely cancel on getting SS and SD as agreed upon. And it routinely would fall on me to break the news to them since W was working or in school. It always was hard to see their sadness and despair at feeling abandoned and unimportant.
I guess now I realize it was me being taught the valuable lesson of stay true to your commitments to your kids. If you say you're going to pick them up... you better do it barring some type of bona fide emergency.
It's also what motivated me to confront my W about her absenteeism a few months back. I found myself right back to be consoler of kids that I'm not mom or dad to. The bond is different.
If she makes this a habit I would encourage you to confront her. Not mean or with anger, just informational... "When you do X, son exhibits the following behaviors; I thought you should know."
Married 6 together 8 Me:38 W:31 second marriage for both SS12, SD10, S6 Bomb: 9/8/11 (day before our 5 yr ann) W moved out: 2/18/12 D final: 11/12/12 Share S 50/50. Spend as much time as I can with SS & SD