Meaning of the doll in The Boy

Braums murdered that girl as a child, and so his parents faked his death because they were either afraid of him or too afraid to lose their son, (probably a bit of both). So they raised him in secret in a sort of pseudo-denial, so that they wouldn't have to lose their son for something he did when he was 8.

Stacey Menear (The Boy Writer) "explained" the Twist Ending Cosmopolitan-interview

So you had multiple endings in mind at one point.

I did. The doll was going to come alive. That was the initial thing, was the doll was going to come to life and chase them around in a scary way. It just never felt right to me. I played around with the doll breaking and a spirit comes out. I think the reason I chose [the ending I did] was going back to the '70s films that I loved so much. There's this one called The Baby and another called Bad Ronald, and they're sort of these strange horror movies more than straight-up scary [ones]. I think horror movies now are mainstream. They never get really weird; they'll get really violent, but they'll never get strange.

Half the fun of watching a horror movie like this is trying to guess the ending. What's the secret to artfully planting clues?

It's a balancing act. I did the draft, got to the ending, discovered the ending just like the audience would, and then it was a matter of being like, "OK, there's a guy in the wall — how could he live there?" I don't know how much this plays to the audience, but when she's putting his food in the freezer, that's supposed to be the place where [real Brahms] gets his food. So, stuff like that. It's just reverse engineering, essentially.

Were there any pushbacks on the script? Was there a thing you had to fight to keep in the movie?

They talked about getting rid of her losing a child; they didn't think that was necessary at one point. And that was so important to me, because it makes her staying there make sense. It's still not an easy buy that she would stay there. But I think it's better if you understand that she lost a child, she's going through something, and she sees something in this doll, in this existence in this house that can fill that void that she's feeling.

Why do you think Brahms's parents needed to kill themselves? Out of guilt?

Yeah. They wanted to find him a match. I don't totally know if this is supposed to be his girlfriend or his mom — I think it's a messed up in-between. But, yeah, I think they felt like they had fulfilled their missions and they couldn't stay there any longer. This was never planned. They hid this kid to keep him away from the police, and this sort of happened over time, and then it got to this point that they're horrified at what has happened and what they've become. At one point in the script, Mr. Heelshire says, "It happened little by little, then all at once."

Now, why was there a doll and why were his parents (in my interpretation) forced to live with it treating it like their real son?

I came away from the movie with a very specific impression as to why the doll and the pretense. It was a way to atone to their child for hiding him away from the world. It seemed to me that while they decided it was a good thing at the time to hide him, it turned out to be a bad choice, either he was irreparably messed up from killing the girl or hiding the little bastard in the walls made it worse. Regardless, they found themselves caught up in this nightmare of pretending to the world that their child was dead and secretly interacting with him in this frozen mindset. He couldn't come out into the real world, his parents would face lots of legal problems, and he couldn't be left because of his psychotic tendencies/age/inability to fend for himself. When faced with their infirmity and the realization that they couldn't any longer and likely were tired of the charade, they decided to simply walk into the lake. Problem They loved their child too much and felt too guilty to leave him to die but not too guilty for fess up. So now he's someone else's problem.