M. Tomasello and usage-based theories

Came across a cartoon which rather reminded me of the whole usage-based theories like those popularized by Michael Tomasello. Essentially, in usage-based theories, very young (less than 4 years olds) do not have the kind of linguistic competence that adults do. As an example, while an adult might (in her/his mind, (if (s)he has one)) interpret the sentence "Mom shuts the door" as [N]_subjNP-[V]_V-[D-N]_objNP, a young kid would only see this as something like [Mom]_N?-[shuts]_V?-[the]_?? [door]_N?. That is, nouns and verbs are tied to each other, so that kids will not readily go from "Mom shuts the door" to "Johnny shuts the door", since the kids don't really learn about NPs and VPs, but about words like Mommy and shuts, and all they see is that Mommy comes before shuts. What about adults? MT quite readily accepts that adults do have NPs, VPs and all the rest. So how does the child go from Mommy to NP? Here's how the answer looks like Here's an excellent debate between Stephen Crain and Mike Tomasello at the BUCLD 2004. Judge for yourselves.