We have had experience with multiple bedwetting alarms. Some alarms make a loud noise. Others make a loud noise and vibrate. Some have to be sewn into the underpants. Some have a clip. Some have cheesier connections. The long and short is that we like the Malem Ultimate Alarm for its ease of connecting to the underwear and the child's jammies. It vibrates AND makes a noise. But let me state this. NO bedwetting alarm is effective without waking the parents up to then wake up the child. The parent must ensure that the child reaches consciousness when the alarm went off. This model has the strongest connection to the underpants that we have seen--helping to ensure that the alarm doesn't fall off, and while it's not the loudest alarm we have heard, if you are close enough the child's bedroom or use a monitor, it can wake you, if not them. It secures to the jammies with a safety pin, which has not proven dangerous. So that's our recommendation. Try The Bedwetting Store for a good price.
We have had better luck with matress protectors--those lovely, sometimes noisy, waterproof mats that fit under the child as they sleep. So far to date, Sears has the best model. For $10 in their home section you can buy a very large mattress that doesn't roll around too much as the child tosses and turns. It also is the only mat the kids haven't peed over the sides of. It is comfortable enough to use OVER the sheet, so it's the only thing you need to wash in the morning. It covers most of the twin mattress. Good luck and persistence should win the day!