The boy died from severe head injuries after an incident involving furniture at the store on Monday afternoon, Thames Valley Police said.
Three ambulances were sent to the scene and he was removed to the Royal Berkshire Hospital where he later died.
He was pronounced dead in hospital just an hour after the incident which happened at 4.15pm, Thames Valley Police said.
"R.I.P Kaden love you millions you will always be in our hearts".
"As the enforcing authority for health and safety within retail premises, we will be working closely with the police, who are at present leading on the investigation".
Police said the death is being treated as unexplained but not suspicious.
A date for the post-mortem examination has not been set and an inquest has not been officially opened yet.
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Outside the closed shop police officers interviewed witnesses and ordered onlookers to keep moving as a large throng of people gathered outside the shutters of the multi-storey clothes shop.
A unsigned card left outside the store read: "RIP little man".
"After treatment at the scene he was taken to the Royal Berkshire Hospital still in a life-threatening condition".
In a statement Topshop, which is owned by the billionaire Sir Philip Green, said: "The Topshop board together with thousands of..."
Detective Superintendent Paul Hayles said: "We are supporting the boy's family to assist them at this very hard time".
Topshop has said its thoughts are with Kaden's family and they were "investigating all the events surrounding this extremely sad situation".
It comes after a four-year-old boy was killed when an 18-stone mirror crushed him in a shop changing room at Bicester Village in Oxfordshire in June 2013.