Broadly speaking, enjambment reflects the free-flowing nature of the poem. The poem is written in free verse, with no rhyme scheme, meter, or stanza form to contain it, and this looseness evokes the comfortable, intimate relationship between the speaker and his mother. Enjambment adds to this effect, letting the poem unfurl down the page freely and pulling the reader forward through its many lines.
Enjambment is also interesting in this poem in large part because of the way it is in tension with the use of punctuation. Pham often foregoes conventional punctuation at the ends of lines. Without punctuation, a line will seem to carry over even when it has actually reached its syntactical conclusion (essentially, the end of a clause or phrase) or at least a pause.
Examples of this abound. In lines 1-3 ("I know now [...] war and exodus"), conventional rules of grammar would call for a comma following the dependent clause "as I did in my childhood wonder" as well as a comma or em dash following the word "paradise," as the following line is also a dependent clause. These punctuation marks would indicate to the reader that they should pause and take a breath at the end of the line. The passage would have looked like this:
I know now, as I did in my childhood wonder,
that my mother dreamed of a paradise—
one unbound by war and exodus.
Instead, Pham chooses not to employ punctuation in either of these situations, pushing the reader to continue without pause across the line breaks—and making the lines appear enjambed. Yet, at the same time, because many of the lines are syntactically complete (basically, they contain discrete phrases with implied pauses at the end), the reader may choose to pause at the end of a line anyway.
The result is a kind of tug-of-war between the reader paying attention to syntax (the arrangement of words) or paying attention to a lack of punctuation.
Another example of this comes in line 4 ("On the living room carpet we sit"), which is not only grammatically complete, but is in fact its own sentence—once the reader has continued on to line 5 ("I pluck her grey hairs and ask:"), it will become clear that a new, separate sentence has begun. For this reason, line 4 might be interpreted as end-stopped even though it looks enjambed.
Ultimately, the choice falls with the reader to decide how they're going to read these lines and whether they will pause at the end of a syntactically complete line despite a lack of punctuation indicating that they should do so.
Other places in the poem employ more conventional enjambment, however. In lines 30-39 (I picture her [...] longer call home.") for example, enjambment allows a long sentence to be broken up over various lines, and the pauses in lines created by punctuation all fall within lines rather than at their end. For this reason, this passage has a flow to it that those earlier stanzas lacked. The reader soon learns to trust the momentum of the enjambment, knowing they will be given ample opportunity to pause within lines, rather than having to figure out whether white space is implying the end of a clause or sentence, as in earlier examples.