Certainly both a precocious and perspicacious young girl, Scout may have realized the symbolism attached to mockingbirds after witnessing the trial of Tom Robinson and then having read the editorial in the Maycomb Tribune that followed the "Colored News" with the obituary of Tom in it. Scout remarks after having read it,
Mr. Underwood was at his most bitter...[he] didn't talk about miscarriages of justice, he was writing so children could understand. Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom's death to the senseless slaughter of songbirds by hunters and children....
Thus, Scout could have a "vision" of Tom as a mockingbird, for her father has told her that it is a sin to kill mockingbirds and Miss Maudie has instructed her that mockingbirds do not do anything but bring cheer to people's lives with their songs, just as Tom Robinson did no harm, and he brought cheer to Mayella's life. Likewise, it is entirely possible that Scout understood that Boo Radley, too, is like a mockingbird, meaning no harm to anyone or anything.
I am not sure where you got the idea that To Kill a Mockingbird by Harper Lee has anything to do with literally killing--or even dreaming about killing--mockingbirds, but there is nothing of the sort to be found in this novel. The idea is first presented by Atticus Finch when he finally gives in and allows his children to get air rifles for Christmas. He tells his children that they can shoot and kill several kinds of small living things, but the one thing they must never do is kill a mockingbird. That is a sin.
This warning is the only literal (physical) reference to killing mockingbirds. Every other instance in the novel which refers to mockingbirds (or songbirds) is referring to them as a metaphor--a symbol of something. After Scout's father delivers his warning, Scout is a little confused about why other birds are okay to kill but not mockingbirds, so she asks Miss Maudie about it. This is how their conversation goes.
“Remember it’s a sin to kill a mockingbird.” That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it.
“Your father’s right,” she said. “Mockingbirds don’t do one thing but make music for us to enjoy . . . but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.”
So, the two qualities which separate mockingbirds from, say, crows, is that they provide us something beautiful in the form of a song and they do absolutely no harm to us.
Two people in this novel can also be described as mockingbirds (metaphorically, if course): Tom Robinson and Arthur Radley. Tom has done nothing but trt to be helpful, even to a poor, lonely white girl who needs some help and even though he knew he might get in some trouble for doing so because he is black. He did a beautiful thing and harmed no one, yet he was literally killed because of it. Mr. Underwood writes about Tom's death in chapter 25 and compares it to the the "senseless slaughter of songbirds by hunters and children. ”Arthur "Boo" Radley did nothing but give the Finch children some beautiful things and try to help them in all kinds of ways throughout the story. Nothing literal happens to him, but if he had been part of a trial, the public attention would have figuratively killed him.
In both instances, killing a mockingbird is a metaphor; in no instance does Scout dream or have visions of killing a literal or figurative mockingbird.