Dont ask.
IF you dont have anything planned let her go. You cant control her.

Id say something like:
"Sure, I have nothing planned those day. If we agree on a rolling schedule around the kids you wont need to ask me anymore."

That is unless you have laws around Right of first refusal.

Me: 34
Stbxw: 30
D:5 D:3
Mini bd: May/June 2019
Married: Aug 2019
BD: 6th Dec 2019
OM Confirmed: Feb 2020
March 2020: I filed for D
Waiting for D to be finalized and W to move out end of January 2021