So, do you have plans for Friday, since you know your W makes plans every other weekend?
If you want the girls, then keep them. If you have plans.....great. But don't try to control what your W does. In other words, don't tell her you have plans, hoping it will keep her from going out........and don't keep the kids to get on her good side. Do what you really want to do, b/c she's going to whatever she wants, anyway.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!