Most children who wet the bed are deep sleepers who don't sense the usual signals that the bladder gives the brain to tell it to wake up and go. Other children have small bladders and can't hold the whole night's urine in. DDAVP works temporarily because it causes less urine to be produced overnight. But as soon as the child stops taking it, the kidney goes right back to making a less-concentrated urine in the usual amounts. Thus, I usually don't recommend using DDAVP for long periods of time. It is best used when there is an important night or couple of nights when you don't want the child to wet the bed (sleepovers, overnight camp, vacations, or outdoor camping). It's not a true cure for bedwetting.
If you and your child would like to try to resolve her bedwetting permanently, you should try one of the commercially available bedwetting alarms. The have a very high success rate, and only a small relapse rate once you stop using it. They cost about $60 to $90, and are available through most medical supply companies.