They were complicit in her disappearance simply by their neglect of their Children's welfare.
While I am saddened by the plight of any Parent who loses a child under any circumstances it doesn't mean that we cannot be critical of the McCanns and their approach to Parenting, especially considering that not only did they leave a 4 year old child alone, but also effectively with the responsibility for two younger children.
Whether they were checking back every 30 minutes or not, it is highly irresponsible for any Parent to leave children of such a young age unattended while they were out socialising, especially in a foreign environment.
The Tapas bar was 120m away from their Apartment, this is the same as going to the Pub at the end of your street and leaving your young children alone in an unlocked house without any supervision for extended periods of time.
Even when they and their friend checked on the Children, one of the friends only listened at the window on the first occasion and only saw the Twins on the second, yet it was half an hour after that Kate McCann found Maddie missing.
There are some pretty serious discrepancies in the accounts of each of the Friends as well as the McCanns and the Resort Officials, some of this we can put down to the panic of the Parents and friends, but it does imply that their is some discrepancies in the accounts that are not accounted for by emotions.
The ironic thing is that Praia da Luz has Child-Minding, Creche and a Nanny service that was available at the time to the McCanns, yet they decided against it presumably because of the cost involved.