Authorities said the youngster was seen to be on the float as he was taking part in one of the many parades marking the US holiday - celebrated on the first Monday in September.
It is not clear if he fell from the float or was walking alongside it when he was hit.
"We were watching them, and the next thing you know, you just hear shrieking and screaming," one witness told The Greeley Tribune.
"The mum was just frantic, of course, just screaming and crying."
This memorial has started to form along 7th Street for the boy who passed away when he fell off the parade float earlier today in Windsor. #KDVR pic.twitter.com/JlW56Uzemo
— Emily Allen FOX31 (@EmilyAReports) September 3, 2018
Local news outlets reported that authorities, who arrived at the scene, appeared to focus on a Toyota Tundra pickup truck that had an ad for Generations, a Christian church in the town.
The boy - whose name hasn't been disclosed - was taken to hospital but later died.
The investigation into his death is ongoing.
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