I don't think lack of obvious cause of death necessarily means her death was an accident or unintentional or not criminal.
Yes, we have seen these cases where the child was beaten to death, or strangled, or stabbed.
However, we have also seen positional asphyxia, when the child was unable to free themselves.
We've seen suffocation while attempting to silence and/or assault the child. We've seen intentional suffocation with a pillow.
We've seen all sorts of causes of death that didn't leave marks. I don't know the laws over there. However, in the US it's generally an additional charge if you "accidentally" kill someone in the process of committing another crime. (Rape, kidnapping, robbery, etc.)
I'm looking at prior cases like this and thinking that this death occurred during/after another crime. I just hope she did not suffer.