Why would there be mucus in a child?

If you are worried take the child to a doctor with a sample of the stool. Everyone has mucus in their stool to some extent. The bowel is lined with mucous membrane which helps the passage of the stool. Sometimes it becomes excessive if there is an inflammation, or the bowel is irritated (for example by spicy food). If the child is healthy and apart from this the stools are normal and the mucus is not bloodstained, forget about it.