At 9 and 11 kids are prone to doing stupid things on bikes in the woods. It’s part of being a kid. This is why we don’t let them drive cars or fly planes.
If you’re out caning it around in the woods and you encounter a 9 year old on a bike doing something unpredictable then your actions list should involve having your own independent accident to avoid hurting the child, and making sure the kid knows that things are ok but he should not be doing backflips/mimicking danny macaskill/pissing whilst riding*.
At this point you should MTFU and get on with your life. If someone wrote to me demanding money because my kid caused them to fall off their bike, I would be on their doorstep ensuring that if they ever come within 500 meters of my children again, the Lord himself will need a telescope to help them find their testicles.
If you’re on terrain a kid can ride, then ride it as if there is a kid riding around the next corner on one wheel.