Evening William, unfortunately the owner is always liable of whatever happens on his land or using his belongings.
For example, you lend you car to someone, they crash it, your NCD will take the hit (not theirs - although the crash will be fully covered).
Same here for your land. Unfortunately you’re liable for anyone that sets foot on your land (even maliciously) and although very rare (luckily) accidents may happen.
Note that you can only be held responsible of issues that have to do with the land like slipping on the ground and sprinkling an ankle or breaking ones wrist. If 2 scouts get into a fight and injure themselves that’s not directly your liability for example although you could still get into problems if the land is remote and someone gets hurt and judge that first aid were too slow to show up.
All that to say that, even if for free, you’re always liable (unfortunately) and cannot get them to sign a document that would opt you out from responsibility (a massive misconception but don’t be fooled, even a discharge from a doctor is almost always worthless - almost).
Hope that helps?