My contract works the other way around - it says failure to arrive within 15 minutes of your scheduled time jeopardizes care for the day as I have an obligation to all of the children in care to maintain the schedule. This used to apply to when I had to walk kids to school as in we had to leave at a certain time and couldn't wait for late people but in your case applies to the outing - did she know there was one - not that that matters. Leave a note on the door next time saying gone about our day sorry you missed us, see you tomorrow.