Not sure what breed of ducks you are talking about. It's best to mention it when starting a new thread as many others won't have read what you wrote before and/or won't remember what breed(s) of duck you have.
Some may get the urge to fly away and some may not. If they can fly the only way you are going to keep them from doing so is to clip their wings.You only need to do one wing. If you are worried about how it will look I believe their is a way to clip the feather so it doesn't look as obvious when the wing in folded. Many people keep ducks and chickens together.