I don’t think you can. Both parents have the right to private conversations with their children. I would suggest that you talk with her before though and let her know that you are not going to speak badly about her to the children and you expect the same. Let her know that they are children and even though custody is about them, it’s not necessary to let them in on all the nasty details. Expect her to talk about you. Tell the children that you feel their mother may talk bad about you and you want them to know that you love them and will take care of them, no matter what she says about you, that is all that matters. Don’t leave them out of it completely. Tell them that you will try to explain what you can but that you will not tell them everything. Remember, you are the parent, you know what’s best for your children.
I know that this doesn’t sound like much, believe me, I know. My husband ask his ex for the same courtesy when speaking with the kids and for about a week that worked. When things started to get dirty and lawyers got involved that agreement went out the window. She felt that she could get the children on “her side” by telling them everything they didn’t need to know. She talked constantly about both of us to his kids and sometimes it was all I could do not to lose my composure and scream at her that they are just kids, they don’t understand and they don’t need to know about all of that. She has told them things about me that were very ugly and were simply not true just so they would ask us about them and she could find out our response. Most of the time my husband simply tells the children that he will talk to them about it when they are older and more able to understand. He explains some things to them but for the most part, trust me, they really don’t even want to know. Now, the children are a little older and custody was settled and none of that stuff makes any difference.
BTW - You can record your own conversations with her without her knowledge, but recording another’s conversations without their knowledge or consent is illegal. Also, document everything, and keep a paper trail of any moneys, clothing or supplies bought. We learned this from the very beginning