Firstly, I don't know if she HAS killed her ~ as I said before the cops have nothing else to go on.
Secondly, if she has killed her it may well have been an accident ~ or if she has done it on purpose, it does happen..as unbelievable as it sounds it is more likely a child is killed by someone they know, or a family member, rather than a stranger.
Thirdly, there is no reason why Maddie's parents shouldn't be questioned as supects. As an example, when my daughter was 4 she broke her arm late at night (getting out of bed and tripping over toys). On arrival at casualty I was suspected of harming her myself. Of course I was horrified at being questioned ''Did she annoy you in any way..did you lose your temper?'' but I realise it had to be asked.