X demands full custody because I'm engaged, Part 2

[sleeper]

Short version: X texted me yesterday at work, "The children want to live with me, can we meet after work and talk?" I couldnt because of appointment, offered for her to join me at counselor appointment today that i had already scheduled. Yesterday they would normally come to me to begin my week (shared joint custody, alternating weeks). I responded, "No." She picked up kids from school before I could get there. Energetic phone convo followed. She has them now.
I called police and they made report. Same story, vagueness in custody agreement which lacks specific day of week, time and location for swap. I will call lawyer today and get ball rolling.
I'm really worried how to approach kids. DD totally wants to live with her now, I told her that's not her decision to make but X keeps telling her she gets to decide. DS is not so sure. He says he has issues (things X has whipped up) and will say he wants to live with her in her presence but opines missing me and wanting to spend time just the two of us.
I have taken the high road in all this and feel I am losing.
Conclusion: possibility of mediated meeting at therapy today. Possibility of X taking kids again.