I'm under the impression that they CAN undaestrnd that they are saying it wrong, but lack the physical fluency to say it correctly. That's really what they are learning at this age, I think. In the same way that they are learning the muscle control to write with a crayon and throw a ball. What I find interesting is that they will repeat the entire phrase back to us, even though we are only asking about one word of it. Do you notice that? There's something about parsing sentences going on there.