Well I read the OP and my jaw hit the desk! I was infuriated on your behalf retirementisgreat. But I see later in the thread that you have managed to come to some understanding with your daughter.
My advice, had you not done this, would have been to just show her this thread and the responses you received. Only two (or was it just one?) person responded positively to her request. She would then hopefully understand that her request/demand was ridiculously unreasonable.
And as for "there is no one else who can look after the children for her" - sorry but that is absolute bullshine. She could try asking the children's friends parents if they would mind looking after the children every month for a weekend so that she and her other half could "have a break". I wonder what the response would be.
Alternatively, they could employ a nanny or use someone like "Sitters" - but unfortunately, I suspect that having to pay for childcare, rather than using free care from grandparents, would make the need for a break rather less urgent.
I hope you continue to stick to your guns and not give in to this atrocious emotional blackmail. Needing a monthly break indeed!!! Pfft.
I could accept asking you maybe twice a year, but every month is just beyond the pale!