How would you answer this question?
Find the most complex way to give a full and complete explanation, then for each bit assign a necessity value for how important it is.
Anything less than a complete explanation is obviously going to leave stuff out, so then you can decide where you want to make the tradeoff by looking at all information above a certain necessity threshold (how ruthlessly you want to cut down the explanation).
The quantification of each element of the explanation is going to be relative and arbitrary, but it's still easier to decide how far you're willing to go if you have numbers to base it on, even if they're made-up numbers.
More seriously, I find that when I think I can't explain something I know to someone in simple-enough terms it is because I'm underestimating the audience. Explaining things slowly without resorting to undefined jargon usually does the trick.
What does the 'child' already know that can be leveraged?
Do you want them to have a fuzzy, perhaps-intuitive understanding... or bring them up (however many long steps it takes) to a complicated understanding?
Can the explanation be delivered interactively, confirming understanding of explained (or assumed) bases along the way?
Second, you need practice explaining things to people for whom this is pretty alien.
Third, you need to treat the audience like it has a brain and respect them. Simplifying does not mean "dumbing down".
Fourth, use image-rich language. It makes things more tangible.