Doh.
It isn't really very complicated. If you don't understand something you lack understanding, and if you don't believe something you lack belief. Both of the lacks are because of the same something.
If you lack understanding of the principles of gravity and free fall, you might believe you can fly instead of falling. If you understand gravity, you should believe that you can't fly, but it's completely up to you.
Lack of understanding precludes belief. Saying you "lack belief" might mean you also lack understanding. So there are four possibilities:
1) You understand it and you believe it's true (that gravity for instance, means unaided flight isn't possible if you're a human).
2) You understand it and you don't believe it's true, you "lack belief" (so you don't believe you will fall towards the ground if you jump off a building--tough luck for you).
3) You don't understand it and you believe something else that isn't true (see 2)
3) You don't understand it and you don't believe it because you don't understand. (you've never seen an object falling, in your infantile world you still have to learn about falling objects, and that you will fall over a lot as you learn to walk).
So I think atheists and theists alike should be clear about their meaning. Do they mean they lack belief in the existence of a god because they've never really understood the concept--they are learning to "walk" as it were, or do they mean they've got past the understanding requirement and can form a genuine belief?
If they do mean the latter option, this is not something infants are capable of, so please stop assigning capabilities to people who don't have them. Infants aren't even "people" yet, are they people?