The youngsters fell into the water while playing on a frozen lake, and the ice sheet broke. The incident came as the UK was hit by heavy snow and freezing conditions.
Three young boys have died after falling into an ice-covered lake in central England at the weekend as an Arctic blast sent temperatures tumbling across the UK.
Police said on Monday that the boys - aged 11, 10 and 8 - were pulled from the lake in Solihull in critical condition on Sunday, along with a 6-year-old, and taken to a hospital.
The youngest remained in critical condition at the hospital, police said. "Sadly, they could not be revived, and our thoughts are with their family and friends at this deeply devastating time," West Midlands Police said in a statement.
"Searches of the lake are continuing as we seek to establish exactly what happened and if anyone else fell into the water."
The youngsters reportedly fell into bone-chilling water when the ice they were playing on gave way.
The incident came as the UK was hit by heavy snow and freezing conditions, causing significant travel disruption, on the eve of a national rail strike on Tuesday that was already expected to bring the country to a grinding halt.
London Stansted airport warned of disruption over the next few days due to the extreme weather conditions.
"Our runway is temporarily closed whilst we undertake snow clearing," it added, with many flights cancelled early Monday.
The airport is the central hub of budget airline Ryanair, which also cautioned about disruption to its flights at Gatwick, south of London.
"Due to ongoing severe snowy weather across the UK, runaways at Stansted and Gatwick have been temporarily closed tonight (11 December), disrupting all flights scheduled to depart Stansted/Gatwick during this temporary closure period," it said.
Both airports were open on Monday, but passengers were told to brace for delays.
Dozens of stranded passengers posted videos on social media showing snow-covered runways and planes stuck on the ground.
More than 50 flights were also cancelled on Sunday at Heathrow, the UK's largest airport, due to freezing fog.
Train and bus services in London were also severely affected after the dump of around four inches (10 cm) of snow overnight, which also forced the closure of parts of the M25 orbital route around the capital, the country's busiest motorway.
Some schools were also shut.
The UK has been experiencing a cold snap for several days, with temperatures dropping to -10 Celsius degrees (14 Fahrenheit) in some areas, although the Met Office said the temperatures were "not unusual for this time of year".
The service has issued yellow alerts for snow, fog and frost in several areas, including southeast and southwest England and the north of Scotland.