I've organised several haiku contests or workshops for children and found the best way is to avoid the urban myth and red herring that is "5-7-5".
Children are really good at writing non-rhyming poems if given the chance.
In a project where I use renga (linked verses started by a haiku) I've found children have written some of the strongest verses.
If you hold another contest, avoid falling into the trap of thinking it's 17 syllables for a haiku. Let the children have fun being challenged, they do like being stretched beyond their perceived capabilities.
For a simple overview of haiku, approved by many Japanese, British and American haiku writers (and educators) check out:
all my best,
Alan, With Words