I finally sent an email letter stating my protest of her poor decisions and actions in this -- for whatever that's worth. And I copied my L.
If she keeps that up, I know that's a factor in granting custody. I was told if I did stuff like that, keeping info from X, that would be a factory in custody. If she keeps that up, I would discuss with your lawyer about possibly changing custody to you being primary. Keep a chart or something on this kind of stuff. Plus, it's just poor parenting; kids need both their parents!!!
When they get older, one good thing is both my kids' schools have email lists that you sign up for and they email you with dates like that. Also, do they have a website--the preschool? If not, maybe you could suggest it so they can post important dates on their website? I'm sure you're not the only parent that's happened to...unfortunately.
