Experts say it's mostly a matter of physical development and maturity that causes them not to be able to hold it until morning. It's something they'll have to grow out of in other words.
However, there could be other culprits too, ones that you CAN control. Doctors and nurses say processed foods and dairy, including milk, can irritate the lining of the bladder, causing more urine to be produced and released. Foods with coloring and preservatives specifically.
You should avoid feeding your child foods with the five C's: caffeine, carbonation, citrus, chocolate and extra vitamin C. They also tend to increase urine output.
Bread may be another culprit because of a preservative widely used in it: calcium propionate. When the additive is removed from a child's diet they tend to stop bedwetting. Try buying specialty loaves such as French or Italian bread and discourage your children from drinking juice or sodas.
The best way to pinpoint exactly what foods may be causing a problem is to do a three week elimination diet, removing all foods from your child's diet that could be causing bedwetting. You can reintroducing them one at a time and watch the result.
Consult your pediatrician before making any major dietary changes. Of course, there can be dozens of causes of bedwetting. Looking at your child's diet is certainly a legitimate place to start.
You can also get more ideas from Good Nites.