Dont forget, that the "kid" if under age (cant quite remember it now 14 i think), MUST be supervised by someone over the age of 21.
As far as Liability is concerned...as an example...if I came around your house for dinner, sat on a chair, and it collapsed...I could sue you for any injury or mental anguish etc...if I was a paper boy, delivering your local rag, came down your path and one of the flagstone was loose and I fell, injuring myself...I could sue you...even if I wasnt supposed to be there...I could still sue you...it's called 3rd party liability.
I would make sure that if any child that isnt yours, has their parent or guardian come round and check that they are happy with teh area and what will go on there and have them sign something...I'm not sure how much weight that signed paper might carry...but possibly more than an unwitnessed "Yeah thats fine.."
Regarding distances...
It is an OFFENCE to discharge any airweapon or firearm, within 50 FEET of the center of The Queens Highway...to me that means any road, footpath etc.
It is an OFFENCE, if any projectile, leaves the boundary of the property.
There is no legal ruling as to distance from another property...I used to target shoot in my parents back garden when I was a kid (very elaborate back stop)....the people who lived out teh back...stuck their head over teh top and complained to me to stop as tehy didnt like it.
"What dont you like?" I enquired....they hadnt heard me shooting (elaborate back stop) but had seen me, thats all.
I expalined to them that what I was doing was perfectly legal...I had an excellent back stop...was deliberatly not shootiong in theor direction...tehy said that they doubt teh Police would see it that way...so I said..well...actually...they will...so...along came PC plod.
One was at teh neighbours house....my Father welcomed teh copper in...i showed him what I was doing...teh back stop etc...he siad they would be concerned about me missing...so I told him to watch..while I shot 5 matchsticks...individually at 25 yards.
He radioed his collegue to ask if he had herd anything...No...that was it.
I wasnt casuing any unreasonable noise...didnt pose a risk...end of.