It is unusual to begin a poem with a dash, and it is easy to jump over the dash and straight to the words. Therefore, it's helpful to have our attention drawn to this dash so that we can think about what it might mean.
A dash indicates a break. In this case, it could mean the speaker is taking a long, deep breath before he launches into his longish narrative about the little girl who counted her dead brothers and sisters among her living siblings. It could also suggest that there is a part to the story—a preamble—that the speaker is leaving out, deciding instead to start here.
In either case, it gives us pause and puts more emphasis on the three words in line one: "A simple child." This is a key line, as the speakers wishes us to understand that the wisdom he gains come from a child. Wordsworth often asserted that children had an insight and purity of vision that adults have lost.
The word "simple" has two meanings: it can mean plain and clear or it can mean not very intelligent. As the poem begins, we don't know which the speaker means, and it only becomes evident to us that the speaker means the first as the poem unfolds.