Actually, "natural means" is something we have to learn. It's not natural.
Which means that the child, when it hasn't learned it, doesn't believe it. (If we follow the same reasoning about "implicit atheism"). The child doesn't believe the world came into existence by natural means, i.e. he/she is a "implicit" theist. (I'm being sarcastic of course. To be serious, the whole "implicit" thing is just silly, implicit this, implicit that, there's no implicit when it comes to null values. I have 30 years experience of relational databases and know how null values work quite well, and there are three states for a truth value in a database, false, true, and null. Null means, "no value" or "unknown", which is not the same as false, or true, but is just the no-value value.)