My theory is this:
They wanted to go out on the lash and gave their kids some sleeping tablets so that they would behave themselves and sleep through the night. Maybe they both gave Madddie a tablet by mistake or something, maybe Maddie was already unwell and this is why the tablets proved to be fatal for her. The friends at their table went to go check on their own kids and in doing so checked on the MCCanns as well. This is where it all went pete tong and actions were taken to hide any evidence. Things got out of control, the media got involved, and by this time they were past the point of no return, so had to keep up the appearance that it was a kidnapping. I think that over time this event has turned them in to two very bitter people with a massive hole in their lives and a big black mark on their conscience. They did everything they could to hide the truth because although they didn't murder their daughter as such, that's how the press would see it and their lives would be ruined. I don't think they killed their daughter, I think it was an accident either due to carelessness, neglect or blatant selfishness.
That said, if you read some of the things all over the internet I'm surprised we haven't had anyone play the Voodoo card yet. The amount of far fetched bow locks surrounding this case is unbelievable. Like the fridge for example. Who the hell blogs about a broken fridge in their resort room? I mean come on, really?