Children who wet their beds need to learn that they have control over their brain and their bladder.
Talk to your child and explain a little about how their urinary system works. There's no need to give them a full anatomy & physiology lesson, but a brief overview will help them understand what's happening... and why.
Give simple explanations such as how their heart pumps their blood around, and their kidneys work like "washing machines" to clean their blood, and how the bladder is used to store their "wee".
Ask your child this question: "If you had to wee right now, how would you know?" Some children will probably reply that they 'feel' it. Some may say they know they feel it because their brain tells them.
If they don't mention their brain, you should point out the connection between their brain and their bladder. Show them this diagram.
Another important aspect of talking about this problem is that you'll let your child know you're really interested in their happiness and well-being. Although it may seem unlikely to you, there's always the possibility that the child feels unimportant, or they may feel that wetting their bed makes you "dissapointed" in them. Of course that's not true, but the child needs your reassurance and support.
Talking about this openly without causing embarassment may help to resolve the problem more quickly. It can give the child the assurance that this is a "team effort", and that everyone is keen to see them happy, relaxed, confident and dry.
Best of all, by talking about bed wetting, you'll open up doors to talk about lots of other issues as well. Your child will know that no matter what, they'll have someone they can trust to turn to whenever they need guidance or support. How good is that for everyone?