Bad dreams that sometimes may take the place of nightmares are not uncommon in children. They may come either because of sleeping after being very tired or due to certain inner fears that get projected in dreams.
You can discuss if the child has fears and reassure him. Make him / her sleep with some mantras or simply invoking peace as he is slipping into sleep. Normally they simply pass away as the child grows up.
Affectionately,
Alok da.