It is thought that he was playing in the sub-tropical indoor pool with a group of other children at the Wiltshire holiday resort when he became unwell.
Emergency services were called to the holiday park at 3.39pm on Saturday.
An eyewitness told the Gazette and Herald that there had been "some very intense resuscitation attempts by staff."
Another told the Wiltshire Times that three other children were in the water when the youngster was taken ill.
Center Parcs did not confirm the age of the boy, but said in a statement: "We are deeply saddened by the tragic news that a young guest passed away after becoming unwell whilst at Longleat Forest.
"Our thoughts are with the family at this extremely difficult time. We are continuing to offer our support to the family and to our employees who provided immediate assistance."