Emergency services were called to a designated swimming area at the lake, located south of Aachen, at around 6pm on Thursday, according to German newspaper Bild. Their parents had reported the children missing.
The ambulance service, alongside members of the German Lifesavers Association and Technical Relief Agency, were able to retrieve the children from the water, where they were then taken to hospitals in Cologne and Aachen.
Despite the best efforts of rescuers, though, the children later died in hospital.
Prosecutors in Aachen are now investigating whether the deaths were the result of negligence.
