"This joke could have easily killed someone".
Horsepuckey! Think about it: Let's say the water baloon has a pint of water in it; that's one pound. Let's say further that the kids idiots were telling the truth and they were between 300 and 400 ft above the ground. Newton says earth's gravity accelerates objects at 32 ft / sec^2; this gives the 1 pound object a speed of just over 100 mph at impact.
I volunteer you to be the target on the ground in this contest! Let's see: one pound object, about 6" in diameter, moving 100 mph ... I think you would get major damage with a body hit and possible death or paralysis with a head or neck hit.
Bottom line: real live true ballistics do support the statement "This joke could have easily killed someone".