Terri Rickards, a Harlow mother who was at the park, described the scene on Facebook.
She said: "This is just devastating news. My two little ones were there today, both under the ages of five.
"We did not allow them on this bouncy castle as it just didn't look safe. The winds were just far too strong! I can't believe what I'm reading, such sad news. RIP little one."
Meanwhile on Twitter, Yvonne Hazard, who was in a nearby restaurant, tweeted: "I was in the Toby opposite Harlow Park when the tragedy happened today. Just awful.
"A lady who came into Toby's saw it rolling over & over. We was all praying no one was still in it. Such a tragedy."
Raindrops spatter the camera lens in the footage, uploaded to YouTube by Your Harlow Films, as stormy weather continues to buffet the park.
A spokesman said the girl had serious injuries and was in a critical condition, but police later confirmed she had died as a result of the accident.
It is thought she was taken to Princess Alexandra hospital in Harlow, where she later died.
Officers are working with the Health and Safety Executive as their investigations continue.
It is the second bouncy castle disaster at Harlow Town Park in less than a year, after another collapsed with children inside in May 2015.
Parents who leapt into the inflatable to rescue their children described it as "something out of a horror film".
Harlow Council officials visited the fair to carry out an investigation into what happened.
A spokesman for the East of England Ambulance Service last night said: "Our thoughts are with the friends and family involved on this sad day."
A spokeswoman for the Health and Safety Executive said it was assisting police.
Last year a bouncy castle collapsed while children were playing on it during a funfair in the park.
Harlow Council officers visited the site following the incident last May and suggested a number of health and safety improvements, including better stewarding of the event.
