This surah speaks about the Jinn who heard the recitation of Quran and accepted Islam. The Jinn are beings made from fire that are unseen. Children know of many supernatural beings that may or may not exist but in this context you could explain that Jinn do exist and may in fact be responsible for some of the unexplained things we experience.
Discussion question: what are Jinn? Have you every heard a story about a Jinn?
Please share your ideas for teaching children the Tafseer of Quran.