10) Araby: Rewrite a brief passage from this story in the voice of the young boy. Use informal style, simple figures of speech, and vocabulary appropriate for a child.

There's some solid guidance for you in the phrasing of the question itself for this task. Joyce's "Araby" is written in the first person, but the language is sophisticated, and, while the linguistic structures often nod to the figures of speech heard in Dublin and other parts of Ireland, it is generally written in fairly formal English. The story is written in the past tense, in a way that suggests the situation the narrator is describing is no longer as it was—"North Richmond Street being blind, was a quiet street..." and so on. Therefore, we can combine these factors and infer that the narrator is telling a story of his boyhood, but is now an adult.
The task here, therefore, is to ascertain:
1. What aspects of the language used in a certain passage make it seem formal, complicated, or otherwise unsuitable for the voice of a child, and;
2. What the key aspects are of what is actually being said—that is, what is the content.
Once we've ascertained these things, we can go about the process of transforming a chosen passage into a style suggesting it was written by the narrator at a point contemporaneous with the events of the story.
There are lots of passages you could choose from the story, but the question mentions figures of speech, so let's choose, as an example, a passage that includes a figure of speech:

On Saturday evenings when my aunt went marketing I had to go to carry some of the parcels. We walked through the flaring streets, jostled by drunken men and bargaining women, amid the curses of labourers, the shrill litanies of shop-boys who stood on guard by the barrels of pigs’ cheeks, the nasal chanting of street-singers, who sang a come-all-you about O’Donovan Rossa, or a ballad about the troubles in our native land. These noises converged in a single sensation of life for me: I imagined that I bore my chalice safely through a throng of foes. Her name sprang to my lips at moments in strange prayers and praises which I myself did not understand. My eyes were often full of tears (I could not tell why) and at times a flood from my heart seemed to pour itself out into my bosom. I thought little of the future. I did not know whether I would ever speak to her or not or, if I spoke to her, how I could tell her of my confused adoration. But my body was like a harp and her words and gestures were like fingers running upon the wires.

Let's break down, then, the key content points of this passage. First, on Saturday evenings, the boy would go to market with his aunt in order to carry some of her parcels. This is straightforwardly written enough. The following description of the people the boy encountered in the market, however, includes language such as "shrill litanies" which seems too advanced for a child, as does the figure of speech "I imagined that I bore my chalice safely through a thong of foes."
Next: the boy finds himself saying the name of his crush, Mangan's sister, from time to time in a way that made him feel inexplicably emotional. The descriptions of how "a flood from my heart seemed to pour itself out into my bosom" and "my body was like a harp and her words and gestures were like fingers running upon the wires" are, again, figures of speech a child would be unlikely to use.
If we were to rephrase this passage, then, we'd need to make changes to the language and particularly reconsider how a child would express the emotions conveyed through metaphor in Joyce's text. Some of the actual analogies in the original text, such as that of the chalice, are not beyond a child's thoughts, but would be phrased differently. An example might be:

On Saturdays I'd have to go to market with my aunt to carry some parcels for her. I didn't like the market much because it was so full of strange, loud folk. Made me feel like I was trying to carry a full cup through a big busy crowd about to knock it out of my hands.
I found myself thinking about Mangan's sister a lot, though I couldn't say why. Thinking about her made me feel funny, like my chest was too full.

Try another passage yourself—I hope this breakdown of the process was helpful. You might also consider whether the tense Joyce chose would be what a child would use—how would it read differently if it was written in the present tense, for example?