One of my families is halfway through their 2-week holiday abroad. They have failed to pay me for last week. I sent a polite email yesterday to make them aware, along the lines of Hope-you're-having-a-lovely-time-btw-I-haven't-received-payment-guess-it-was-overlooked-in-rush-to-prepare-for-holiday type thing.
Their response today is that they have 'paid up to date for childcare received' 
How do I politely remind them that when they choose to go on holiday (giving me a week's notice of said holiday) that I still need to be paid? My other families do/have done this as a matter of course and have never put me in the embarrassing position of having to demand payment like this. WWYD?