R.M.
First of all...I am sorry that you are having to deal with such an unpleasant situation.
If I were in your shoes I would be very careful about approaching the "other woman" you have no idea what her mind set is...she might be very receptive to what you have to say but then there is also the chance that she would twist anything you say or do and use it to cause more trouble between you and your ex.
I would just sit back and observe...talk with her if she comes with your ex to pick up the children....listen to what your oldest child has to say when they come home from visiting their Dad.
I just don't think it is worth the chance of stirring up even more emotions or drama by talking to her about your expectations.