Emergency crews are searching for the youngster after reports that three children were in distress in Westport Lake in Stoke-on-Trent.
Two of the children were rescued but one youngster is still missing, Staffordshire Police said.
“The identities of the children are known to us and we are in contact with the parents,” a force spokesman said.
Police, ambulance and fire crews rushed to the scene after they were alerted at around 4.50pm on Monday.
Staffordshire Air Ambulance and the National Police Air Service have also helped with the search operation.
Shortly after 11pm on Monday, a police spokesman said: “Unfortunately, due to fading daylight, the search operation for the missing child at Westport Lake in Stoke-on-Trent will now draw to a close and will recommence first thing tomorrow morning.
“Officers will remain at the scene overnight.”
Staffordshire Fire and Rescue urged people not to swim in open water after temperatures soared to 29.4C in parts of Britain on Monday.
“Even during the hot weather the water is so cold that it’s easy to get into difficulty,” a fire spokesman said.
“If you do fall in please try to relax then when breathing is under control float or swim to the bank.”