Oh man, that stinks. I can imagine how you feel! The thought of the OW around my kids makes my blood boil.
In my situation, I've been gathering as much info on the OW as possible, looking for dirt. Just in case it's possible to legally prevent her from being around the kids- maybe in the custody ruling.
Have you been able to find anything unsavoury about this man? If he's an upstanding citizen that route may be out, though.
For now, I wouldn't say anything.
Me-29 H- 28 Together for 6.5 years 2 kids, ages 9 and 5 D-Day: July 15, 2010.