Children are naturally curious. Let them ask questions. If we don't know, we say we don't know. Read Surah Ikhlas to the child.
Surah 112. Al-Ikhlas – The Pure Monotheism
Author’s note: This a very brief Surah that very forcefully and clearly expounds strict Monotheism, nothing confusing about it such as Trinity, Dualism or any shade of Polytheism.
With the Glorious Name of God, the Instant and Sustaining Source of all Mercy and Kindness
112:1 Say, "He is God, the One and Unique! [He is Unique in His Essence and Attributes and He alone is the Sovereign Law-Giver in the Universe. ‘Wahid’ = One. ‘Al-Ahad’ = The Unique]
112:2 God is Absolutely Independent. [Samad = Absolute, Eternal, Unique, Absolutely Independent Lord, Perfect, the Uncaused Cause of all that exists]
112:3 He begets not, nor is He begotten.
112:4 And there is absolutely none like Him."