I would take it as the child reporting what they heard from the other room. I am not personally seeing any signs of someone telling them alternative explanations to confuse the story at all. Everything seems clear as long as I take it that it's someone in another room listening to what's going on and interpreting the sounds they hear.
So if the child says Mariah woke up crying, then I would interpret that as they heard Mariah crying. I would interpret the uncertainty in knowing for sure whether it was EK or KW as the child being unable to actually see, but being able to hear raised, angry voices, I don't know if they could hear a loud slap through the walls, or maybe Mariah going flying across the room? But if not, I think it would be interpretable by hearing the raised voices and then an increase in the level of crying, then more shouting, and you'd figure that the increase in crying was from Mariah being hit/hurt by an adult.
There was an interesting comment above the one I've quoted, about the actions of EK and KW. If KW was innocent, why not scream blue murder and go and get a neighbor to call 911? Why not scream at EK to leave the home and call 911 as soon as he's out the door and bolt it behind him? But instead we hear that KW told EK to hide the body, and EK does as he's told? This sounds to me as if they both feel complicit in what happened and at that moment in time all they want to do is 'cover up' what really happened for their own protection.
There seemed to be some uncertainty in that speakerphone call over whether or no Mariah was actually dead. That could just be from someone in another room hearing someone say, "is she dead?" And then maybe an answering voice saying, "no, I don't think so..." And then the next thing is hearing "you've got to hide the body!"
So there's more going on, we're getting a synopsis almost. But it does sound to me like a first report from a listening child, and they would need re-questioning to get a few more details.
We don't know when the bloody nose occurred. There's no report of it happening during an attack on Mariah, so it could have happened earlier in the day. The boys could have just hunkered down in their room with blankets over their head trying desperately to block out these sounds, and then in the morning pretending they slept through the whole thing and playing naive so as to not get themselves into trouble. I keep coming back to how would those boys be feeling after hearing their own mother tell EK to hide their sister's body? Wouldn't they be fearing that if they put one toe out of line that might be their fate, too? Especially bearing in mind the allegation about the belt and the bloody nose...they've likely suffered being hit, too. They don't know how they're going to survive this situation.
I think that maybe going to school that day, and then not being allowed back in the home (as it had become part of a crime scene) that night, kept them from being told an alternative story by KW. If they play "we were asleep, no we didn't hear anything!" in the morning, and then by the evening the grandmother is around and KW doesn't get privacy to take the boys aside and feed them an alternative story, then all they know is that the police are around, their sister has been reported 'missing' (but at least one of them heard 'hide the body' and finally they've been questioned, possibly by a police officer with the grandmother present and grandpa listening outside the door, and I think they've just told exactly what they heard. I think the boy(s) were showing extreme bravery at this point to relate what they heard no matter that it implicated KW, their own mother who, from what they've heard, was complicit in the death of their baby sister and the hiding of their sister's body. I would think KW would have been in police questioning at the police station at the time for them to open up this way.