3 1/2 is stille young. But take an appointment with your child's doctor to check with him if your child has nothing physical.

Once the physical is ruled out, you'll be left with nocturnal enuresis. Which is not a matter of training.

The enuresis is not a disease but a problem of development and reflex. The enuretic person has a sphincter weakness quite simply, i.e. a muscular weakness on the level of the collar’s bladder. As soon as a certain accumulation of urine is made feel in the bladder, the sphincter (too much weak to retain the urine) slackening and causes an involuntary loss of wee.