Nearly all organizations these days require some sort of signed waiver to participate in their activities. When dealing with children, you should always secure signed parental permission on a liability waiver as well. The ACA has such a waiver and it is required for all ACA sanctioned events - no exceptions. If you are running a children's trip without accompanying parents, many organizations will also required a signed medical release. The medical release enables a non-related adult to check the child into a hospital for care in the event a parent can't be contacted in a timely manner. As a trip leader, you need to ensure these forms are readily available especially the medical waivers.