Toddler groups and local/cheap/free outings I include in the fees. I charge separately for occasional outings and give parents a permission form which includes the amount due as well as info on what to bring/if they need to arrive/collect at different times. I don't charge extra if we go earlier or come back later than the contracted times. I do ask for the price of the admission ticket and a contribution towards the cost of transport. I usually provide a picnic.
So far, everyone has been happy to pay for trips. I don't too them too often, I know what its like to get endless requests for money from my children's schools. And I do make use of local places that are free - museums, parks & paddling pools, libraries. Fire station and police stations are great for free trips - if you organise a few other childminders/parents to go with you. The children also like the local farm shops (cows, sheep, chickens etc to look at).
If a parent did decline to give permission I would find out why and see if there's anything I can do to change their mind. Or re-schedule the trip for a day that child wasn't present. (all mine are part time so usually easy to re-arrange). Otherwise I don't see any option other than to cancel the trip and return all money paid.
The general rule is that if you are open and the child doesn't attend, whether sickness/holiday/days off, then you charge full fees. Unless you have negotiated a retainer, eg over the school holidays or if parent on maternity leave or child on long-term sickness in hospital/operation etc.