It's inevitable that someone is going to mess with your bike, no matter what you do. In fact, the more obnoxious you make an alarm, the more likely an offender might do something worse to your bike as you might be :cus:ing him off.
I'd proceed with caution with what you do...sometimes just getting a cover for your bike will do the trick.
I was at a picnic this weekend and some guy took his kids and put them on my bike..my cramp busters were twisted all around and the little kids got muddy shoes on my seat. Was I angry??...you bet...don't touch what doesn't belong to you...the bike is not a jungle gym and it's not public property. Simply put, people need to be more responsible. How about asking before you plant your two kids on someone else's property?
Get a cover for your bike...best way to go...