I'm really interested in the case; watched the Netflix documentary as well as loads of amateur sleuth theories on Youtube. It's really hard to wade through all the stuff but in general there are too many inconsistencies in the accounts of the parents and the group of friends they were with. Everyones' story kept changing.
The most plausible theory I've heard is that Madeline actually died a couple of days before she "went missing". The parents used that time to hide and dispose of the body and then staged a kidnapping. The parents were sedating their children at night, Madeleine got up one night disorientated, tripped, fell and died. Rather than face manslaughter charges, the parents just hid and disposed of her.