What's really cool about this alarm is that it uses light, sound, and vibration to wake your child at the first drop. It's also relatively small, and really sturdy, so it'll last as long as you need it to. Maybe best of all is that you don't have to sew anything to make this alarm stay put.

Dri-Sleeper Wetness Alerting Device
This one's neat, too - and costs a lot less than the Malem alarm.

This alarm works especially well for kids who don't do well with a wired model. The pad goes in a pillowcase or under a sheet. 

Malem Vibro-Watch 12 Alarm Vibrating Watch - Pink

How's this for clever? A watch you can set to vibrate and wake your bedwetter without waking your whole family. Good at home or away.