A murder probe was launched after police were called to reports of a knife attack in the Romford area by the ambulance service at about 9.25pm yesterday.
Chesney was pronounced dead at the scene, near St Neot’s Road in Harold Hill, just over an hour later.
Her grandmother, Debbie Chesney, wrote in a Facebook post: “How have we come to this point where kids can’t have a walk in a park without suffering an unprovoked attack?
“If anyone knows anything about this please contact the police with information. We don’t want anyone else to go through what our family is suffering right now.
“This has to stop, there are too many young people having their lives cut short by needless violence.”
No arrests have been made so far, the Press Association reports.
A mother-of-three who tried to help the teenage girl in the park was “shocked by the amount of blood”, her husband said.
Paul Farenden, 54, said his 49-year-old wife Teresa had rushed to the scene near their home to see if she could help after she heard screaming.
He added: “My wife asked if everything was alright and one of the boys said ‘no, my girlfriend has been stabbed’.
“She put her in the recovery position and waited for the ambulance to turn up. I think she tried to resuscitate her.
“She thought it was all over then and it would be alright.”
The death brings the number of homicides in London this year to 18.
On Tuesday, Che Morrison, 20, described by his family as a “loving” young man, was stabbed to death outside Ilford station, east London.
Acting Detective Chief Superintendent John Ross, of the East Area Command Unit, said: “I want to express my deepest sympathies to this girl’s family and friends. Her death is a tragedy.
“I can reassure them and the whole community that we are doing everything possible to identify and bring to justice the person or persons responsible.”
Neighbours said young people often “hang out” in the park, and if they are too loud they are usually moved on without any trouble.
Anyone with information is urged to contact police on 020 8345 3775, tweet @MetCC or call Crimestoppers anonymously on 0800 555 111.