This poem has the rhythm of what we call ballad meter; this means that it is broken up into four-line stanzas, the first and third lines of which have four stresses or accents per line and the second and fourth lines of which have three stresses or accents per line. Ballad meter is also an iambic meter (which means that the predominant foot is the iamb—a group of two syllables: one unstressed followed by one stressed). Ballad meter also calls for an abcb rhyme scheme for each stanza which this poem doesn't quite have; it has an abab rhyme scheme (meaning that the first and third lines share an end rhyme and the second and fourth lines share an end rhyme). Let's look at the second stanza: I will put stressed syllables in bold and divide feet from one another with the "|" symbol:
I met | a lit | tle cot | tage girl
She was eight | years old | she said
Her hair | was thick | with ma | ny a curl
That clus | tered round | her head
You can see that line 1 has four feet, line 2 has three feet, line 3 has four feet, and line 4 has three feet. There is a metrical substitution of a different type of foot called an anapest, one at the beginning of line 2 and one at the end of line 3. These feet have two unstressed syllables followed by one stressed (as opposed to the iambs, which have one of each).
William Wordsworth's poem, "We are Seven," is verse written in four-line stanzas about a discussion he has with a little child regarding the number of children in her family.
Two things seem to provide a sense of rhythm in Wordsworth's verse. He uses rhyme. Except for the first stanza, the author uses an ABAB rhyme scheme, where the first and third lines rhyme, and the second and fourth lines rhyme. The essence of rhyme seems to appear as the reader moves to meet (find) each end rhyme.
The pattern of rhythm that Wordsworth follows lends itself to a rhythmic 4-3-4-3 pattern of stressed beats per line: in other words, except for the first stanza, the first line of a stanza has four beats, the second line, three, and so on. This is the second method Wordsworth uses to create the poem's rhythm.
The rhythm Wordsworth employs provides the poem with a lilting or swaying motion, most obvious when the poem is read aloud, the optimum "delivery" for which poetry is written. It is only by reading poetry aloud that we can experience the many "musical" qualities the poet includes in his verse.