And again another issue with parents ........

Only freebie here is if i am closed.... Otherwise full rate is paid for any days they are not here....you are waaaaay to generous....

I think you have no option but honour the contract you made. I would change for anyone new coming in that either they pay full rate or if you want offer discount, you limit the max number days a year to 10 and if they have more than that, they pay full rate.
It unlikely this current client will sign revised contact but nothing to stop you replacing them with one who will and give notice. If they able afford this much time off, then this might be annual event.

Interested in what you decided to do?

Parents pay full rate to hold their spot at my Dayhome, even on vacation. I've had parents threaten to pull out, thinking they could bully me into caving, but I stuck to my policies and they backed down.
I would advertise just in case she is serious. You should not have to take another cut because they feel like it - if they're on vacation for two whole months I think they can afford it!!!