Just bad journalism I suspect. Story picked-up, through a non-English language paper, they purchased the pictures and made-up some copy to go with the photos.
Fills column inches, parents are "foreign" so not likely to object, and readers happy with a human interest story that can leave them smug that they are better parents.
All sorts of likely scenarios could have lead to this. Mother in water with 2 children, 1 DC urgently needs her attention, drops grasp of other child momentarily, strong current drags baby away. Unable to pursue safely with other child. We all like to think this sort of thing couldn't possibly happen to us, but it does everyday, we all make a poor judgement sometimes.