find out if she is having medical problems. this sometimes can be the root of the problem (TIME)
if not get with the ex- if possible and sit down with her and discuss what is on your mind. Say this is out of character and if anything is wrong.

If she has no reason to think that your ex and you do not attend school functions then I agree that she is out of line. She should not judge others by the mistakes she made...but it is possible, as you suspect, that she feels very guilty about it and wants to "see to it" that you don't make the same mistake. Since she missed so much of your school life maybe she is trying to get what she missed through your kids. She has to be told how it is making you feel and the problems and worry she is causing you. If she shows up for the games and plays could it be because she wants to enjoy the grandchildren's activities and be part of the family. Grandchildren are so special we grandmothers sometimes don't want to miss anything. But of course, that does not explain her picking them up without notifying you. Most of all I hope she will listen to your needs. Some time ago my youngest daughter and I learned to communcate by saying "I feel . I need . Will you please ". This really worked for us when we practiced it.
Good luck and hang in there. You only have one mother and it sounds like she's a keeper!

As many parents suggested , set up specific rules.These rules must be reminded to your mother often for her to fully appreciate them. For example, you have a meeting with your children's teacher on a particular date, let her know in the morning of the meeting and added that you do not want her in the meeting or school.

I guess she is a caring and loving mother/ grandmother, but there must be some boundaries for everyone. It will not only help both of you, but your children and their safety.

I think when members of the family are working together as unit, children and the school personnel are more likely to have a wonderful school year. In sport they often play better and win many games.

Best wishes to you, children,your mother!

My dad was kinda like that....and i put distance between them(there is more to that, but i do not wish to share), he told my child, he did not have to follow my rules, allowed my child to break MY rules....so, i move....but i could not talk to my dad, he was a VERY CONTROLLING MAN....hopefully you can talk to your mom, if not like they said fill out forms at school, and to double ensure, i would write a note to their homeroom teacher's....good luck .

I know I don't have a lot of advice on this subject but one thing you can do. I had to do it with my son's biological mother was sign non release forms. She is not allowed to take J from school or ANY school related function with out my verbal permission. That right there would put a stop to her taking them when she is not supposed too. As for baby sitting and what not I would put a stop to all that until she understood and respected your boundaries. Also I would talk to your ex about his so fanning the flames of your mother's behavior.

Woah!! Set some limits with her. You will probably just have to be honest (nice sit down, before you or your husband lose your cool) where you can be firm with what is acceptable and what is not. My mom can be way to involved with how I raise my kids sometimes, but she does not cross the line. You will probably just have to be kind but HONEST. Eek. Good luck.

As suggested below, I would sit down with her and set clear guidelines of your expectations of her. It would scare me to death to arrive at some function to pick up my children, only to find that they had been picked up by someone else without my knowledge or authorization.

Also, I'm not quite sure what you mean by "she gets all the info on my kids and me." Is your mother providing the SO with school information that you should be getting? Is your mother revealing personal information about you to your ex and his SO? To me, both are unacceptable, and you need to tell your mother that your personal information is no one's business but your own.

This question may seem way out there but, is your mother's health okay?

Whoa - sounds like you need to sit down and have a heart to heart and set up some boundaries.
Have you been able to do that, or are you not at that point yet?
Perhaps, just perhaps taking a UNITED from with you and your ex-husband sitting down with her and laying out what is acceptable and what isn't might work?
Is she normally a reasonable person?