My Ex is using me for child care on their days with the kids. When I have the kids, on the rare occasion I have an obligation, I arrange child care. I give the ex the option, but ultimately the responsibly falls on me.

On my exes days with the kids they've asked me to take the kids atleast 2-3 times a week. I feel like they are taking advantage of me desire to sirens time with the kids.

Should I just be grateful for extra time with my kids? Should I tell my ex to deal with her own problems and not confine to rely on me? I feel stuck.

Couple of points.
1) I love my kids, and want to spend time with them. This is more about boundaries and not being used by the ex.

2) My ex initiated the divorce and actively fought all attempts to reconcile. If we were still married this wouldn't even be a thought, I would re arrange my schedule for them any time.