I do have a permission slip that I get parents to sign every time we go out.
It just basically says:

(dayhome) will be going on an outing to (place) on (date) between the hours of (time) and (time)

Mode of transportation:

Car___
Bus___
Walk____

The fee for parents is:

Fee:___
No fee:___

I (parent's name) hereby give permission to (dayhome) to take my child (name) on above mentioned outing.

I think that's about all that's on there (I don't have it near me right now.)

If I plan an outing and the parent's don't sign the slip and give permission, then it is their responsibility to find alternate care for that day.