Until we find out what all the evidence is that the police have against the parents, there is no way to know if the suspicion of the parents in their daughter's death is warrented. It is horrible to treat an innocent person with mistrust, however. They must question the parents because too many times close relatives are involved in deaths, but they need to do it in a fair manner.
The fact that they left 3 very young children alone in a strange country while they went out to eat is extremely negligent. No fit parent would consider such a thing. I am surprised that the other children were not removed from their care. I think in the US, they would have been.
I am very sorry their daughter is missing and I hope she is found alive. If they had nothing to do with it, they must be beating themselves up for their poor judgment.
Did they say that every parent leaves their children alone at some time? If they said that, I wonder more about them.