This topic exists to make a point. If the universe can be given an honest first explanation that a four-year-old can hold, then the excuse that a subject is too advanced for young children stops working for almost anything else.
It also happens to be the subject children ask about most freely, because nobody has told them yet that they are supposed to find it intimidating.
The universe is everything there is, including you, and most of it is empty space.
The universe is everything there is, and almost all of it is empty space. The biggest subject available still has a first sentence a four-year-old can hold.