Communicate your schedule and let her know good times to see the kids. Don't tell her what is acceptable that sounds too judgemental/preachy. Give her information. Let her know why last minutes plans wont' work. Has this been a issue before, you being too rigid and her too impulsive? If so then it is a 180 opportunity.
She tells you are in denial then you calmly state, "You don't know what I am thinking or feeling right now. If you want to know then ask but please don't assume you know my thougths." This is her mind reading, she fills in the blanks because probably in the past you didn't let her know what you were thinking or feeling- Yes?
She is stressed and you are adding to it. Ask her "How can I help you see the kids more?" Then just listen, if she spews then let it roll off your back until she gives a solution. How would you handle a upset sub-contractor?
Cheers
M22,H45,W45 S21/18D12 Retain faith that you will prevail in the end, regardless of the difficulties and at the same time confront the most brutal facts of your current reality, whatever they might be.