I would say (firmly) on Friday at pick up-if nothing by then, "perhaps there was a misunderstanding, but I required payment this week for the first 2 weeks. In order to come on Monday, I must receive 2 weeks fees, and in the future, fees are due on Friday for the following week".

All my families pay in advance, either weekly or biweekly, none of them questioned me on it (I always paid in advance for daycare for my oldest).