[Retroman]

Well if that was true, they'd still have to be mentally unstable to invite everyone to investigate...and potentially find them out.
Plus it was accidental, so they shouldn't need to worry about being convicted murderers.

And if one parent killed their child, the other one would have had to agree to keep it quiet. If my partner accidentally killed my child I wouldn't want to protect them. But of course, that's just me.

But yea, basically...
They killed their child by accident, so decided to improve matters by lying to the entire world that she'd been kidnapped, and thus potentially go to jail for that too.

Also, if they disposed of the body somewhere..it must be nearby and in the UK. Surely friends and family, or even neighbours and townspeople would be aware of them disappearing out of the country for a while before telling the police their daughter was missing.

And for two regular parents, they've done an awful good job of hiding her over here. Especially considering everyone is aware of their missing daughter.

You can't go around accusing parents of murdering their children just because you're growing tired of not finding out who it was...
And throwing plausible stories around doesn't make them true either.

You're quite possibly contributing to the ruin of two people's names, who have done nothing but been through the trauma of losing their daughter.