I just came across a great video clip which explains, in terms a child could understand, the science behind the unimaginable - time travel, altering the past and future, accessing parallel universes and dimensions to change the present, etc.   All the wacky, kooky stuff.

If you have 11 minutes to bring your brain up to speed with these concepts, check it out.

I should add the qualifier that any child who understands this will likely be on the older side.  And probably considered kinda smart too.