You asked

What do I do if my child who suffers from bed wetting has to go on a class trip?

If your child wets the bed, he may be understandably anxious if a class trip or sleepover is planned because of the potential embarrassment and not want to go.
You should encourage him to take part. Bed wetting shouldn't get in the way of him having fun.
Discreetly discuss the situation with your child's teacher or parent whose care your child will be in. This won't be the first or last time they will have looked after a child who wets the bed.
Teachers will usually have a plan of action and once you know what this is, you can form some strategies with your child so that if he does wet the bed, he won't have to go into embarrassing explanations.
You could also consider using pull-ups while he is at camp.

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