This is an interesting angle on the mockingbird symbolism in the book. Classically, it is Tom Robinson who is compared to the mockingbird, because, as Atticus says earlier in the book, "It's a sin to kill a mockingbird" because it is a helpless, innocent bird that is just living its life and singing its song. To kill it is to kill it for pride and it is completely senseless. Tom represents that definition exactly-he is an innocent man who is just living his life, and he is unfairly and senselessly condemned for a crime that he did not commit, because of the pride and fear of a community, and their racism. Boo is also often compared to a mockingbird because he is cruelly imprisoned by his father and brother, and subjected to mockery by the town and children, when he is really just an innocent man.
So, to compare Jem and Scout to a mockingbird as they walk home, is an interesting thought. They are innocent, beautiful children just going about their business when Bob Ewell takes it into his head to harm them, because his pride has been wounded. For the sake of pride and a wounded ego, Bob decides he's going to take Jem and Scout out, just like a hunter decides to take a beautiful songbird out. Jem and Scout are the innocent songbird, Bob Ewell is the prideful hunter, in this situation. Does that make sense? I hope so! Good luck!