I know I don't usually post to you but something occurred to me when I read your post.
Does your son have an opinion on the baseball etc?
I can see why you don't like your W's tactics, and the way she just signed up for the activity is wrong, BUT maybe there is an underlying thing going on. Let me explain....
My parents split the day after my 8th birthday and my sister was 11.5 yrs old. To begin with we saw our dad at the specified times....but as time went on and things settled down and my sister and I felt secure again.....well then we started doing what kids do and wanted to hang out with our friends and unfotunately the best time for doing that was at the weekends, which was also our dad's time. Our friends all got together and hung out at the weekends and so if we missed doing that we were sort of left out. We would be completely clueless about things that would have happened on the weekends etc and we just felt slightly left out of our friendship groups.
It maybe that your children are saying to their mom that they want to do these things and be with their friends, but they don't want to hurt you and say it to you. She may be covering for them to try and soften the blow. It's not to do with not wanting to see your parent.....it's to do with growing up and wanting to be with your firends. Lets face it, when you are young you don't imagine a time when your parents won't be there at your beck and call. The children don't always understand what a big thing it is for the parents to see them. I mean, if you and your W were still together this wouldn't be an issue, as there wouldn't be such rigid time divisions and these things would flow naturally.
My dad found it very hard to cope with, but he did let go a bit and my mom accomodated him into different times of our week so it worked better for all of us. Sometimes these things just need a bit of tweaking.
If I have spoken out of turn as I don't know your whole history, I apologise.
Saffie me 46 H 46 M in 1986 D20,D18,S16,D13 H's A 01/05 to 07/06 H recommitted to M 07/06 renewed vows 09/06 Going from strength to strength