To be honest... being a single mother of 3 myself and dealing with possession schedules and my exes (2 fathers), you have no real right to ask her the details of what she does with the kids during her possession period IF it is not harming the children.
With custody issues, it seems to really be a battle of "control" of the children. Control on whether or not to allow the children to do this and that... control on whether or not the other parent should be able to do this and that around/with the kids.
When I was going through my custody battle for my son (whom was a baby at the time) his father wanted to start him on baby foods which I was opposed. I told my attorney and she informed his attorney and requested that my son's father do not change any regimen, schedule, etc. while in possession of the baby. However, I was told that even though my attorney and his attorney agreed that this was not an unreasonable request, my son's father was actually in his right to do so if he wanted, because what he did during his possession period, as long as it was not harmful to my son, was his right.
You can not question on who watches the kids, as long as they are a competent adult. You can not question on what she does while with them, as long as it's not harmful. She is in her legal right to do what she wants to do with the kids while in her possession as long as she doesn't put the children in harms way.
I know it's tough to let go of the control over a situation, but if she is a fit mother and is competent enough to make decisions regarding the welfare of the kids while in her possession, then you'll need to let it go...
Pick your battles...