What you describe there is little to no chance of liability
Example...lets use yours...
a teen falling off a semi-trailer bed 30+ feet long and fall on their head and be paralyzed or break an arm or leg, etc.
In order for the teen or the parents of the teen to hold your organization liable, they must first prove liability. They must prove:
1. That the organization had a duty to the child
2. That the organization breached that duty
3. That the breach of the duty was the cause of the injury
They have to prove all three.
What you describe? Someone illegally parks a rig in the lot and some kid crawls up on it (I assume without permission) and does a header on the asphalt? I do not see how your organization could possibly be liable.
Now...change the facts...say you are charging money for parking...and say you are inviting kids to crawl on the cars? And the kid falls...ok...in that case, I see liability.
But not with what you describe
Just because an accident
happens on someones property does NOT mean the owner is liable. The person brining suit must meet the 3 prongs I list above.
What you describe? I do not see it...I mean, you have a rule that makes parking illegal. Someone parks illegally and someone else (the teen) without permission crawls on the illegally parked car? Not illegal. ANd not a basis for liability