Yup all my families pay in advance too (on Fridays by 5pm for the following two weeks) ....I only accept cash or emt and I too have a 3 day minimum
I would tell the family the rules in plain English .... You pay in advance (tell them the amount required so there is no mistake) and if you use an extra day or two then You add it on to the next pay period. Put it in writing and have them sign it and put an expiry date on it ..... Such as " please sign and date this form and return it to me by xxxxx. Failure to return the form by xxxxx will signify that care is no longer required "