I hopped in to say the same thing http://www.amazon.com/Havahart-5266-Activated-Sprinkler-Repellent/dp/B0079GHJVG
Be a real geek and record the fun with Raspberry Pi camera so if he damages the sprinkler you have him(them) for destruction of property as well as trespassing.
Get, or borrow, one of this guy: https://youtu.be/O6oeg0qaI-Q?t=15s
Or, more realistically, something like this