I would absolutely ask immediately for that payment. And at this point I would probably give her notice that if it happens again, you will either be terminating your contract or requiring your payment in advance. I REALLY doubt that she didn't know/forgot that it was time to pay, considering you reminded her of how it all works when you emailed her the last time there was an issue.
I would put this family on a short leash. His behaviour plus her ignorance so far towards your payment agreement is setting off warning bells for me!!!