As William Zinsser notes in his popular writing guide, On Writing Well, the first line in any piece of writing is the most important and keeps the reader moving to the second line and from there through the entire piece. Many ways exist to grab a reader's attention. You could begin with a provocative (attention-grabbling) quote from the novel that either captures some of what the children face—racism, bodily harm—or what they learn. You could find a quote from near the end of the novel and use it to trace back how the children get to that point. Other techniques include opening with a startling fact, an anecdote from the novel that you retell or summarize that captures the children's maturation or character in some way, or a descriptive passage about the sleepy Southern town that shows it really is a microcosm of life from which the children can learn. As Trimble suggests in his book Writing with Style, you can also go for a short lead sentence of four or five words. If you have time, it can be helpful to look at famous leads, such as the opening paragraph of The Guns of August or your own favorite openings. Lee's book is so richly imagined and so rich in imagery that it should not be hard to come up with a provocative lead. One suggestion would be to quote from near the end of the book:
Something crushed me in my costume and I fell to the ground. I was on the ground floundering around...
You could then ask how a little girl dressed up as a ham could mature from being attacked on Halloween, and from there move on to include other characters.
If by "attention grabber" you refer to what journalists call "the hook" or what essayists often call "a motivator," then you may wish to find a short passage in Harper Lee's To Kill a Mockingbird which illustrates the ingenuousness of the children in the beginning of the narrative, a belief which they later abandon in their maturation. Perhaps, the children's superstitions may be your focus for this attention grabber.
Consider, for instance, how in Chapter One, for example, Scout remarks that there is "a malevolent phantom" who lives inside a neighboring house. The children's superstitious beliefs extend to thinking that walking in the Radleys' collard patch will "wake up the dead." Added to this superstition, there is the children's discussion of Hot Steams in Chapter Four. So, if you peruse these chapters, you can find a passage that you can quote as an opener. Then, you can write about how the children's maturity effects a change in such superstitious beliefs. For, at the end, Scout stands on the Radley porch, looking at the neighborhood from an entirely new perspective in which she realizes that many of her childish superstitions and beliefs were immature and incorrect.
You can talk about the racial segragation that is going on in the book.