The girl’s body was discovered by a member of the public in West Park, Wolverhampton, at around 7am on Thursday.
The youngster was reported missing the day before and a post-mortem examination concluded she died from a blunt force trauma to her head.
A 16 and 17-year-old boy were arrested at their homes in the city on Friday and remain in custody where they are due to be questioned by detectives, West Midlands Police said.
Police said the victim’s family have been located and are being supported by specialist officers.
Detective Chief Inspector Edward Foster, from the force’s homicide unit, said: “Following the discovery of the girl’s body, the park was immediately sealed off to allow us to conduct a full forensic search of the area.
“My team are working around the clock to understand what happened to this girl and our inquiries to date have led to today’s arrests.
“A number of people have come forward with information, but I would still like to hear from anyone who saw anything suspicious or saw a young girl in the park overnight from Wednesday onwards.”
The park remains cordoned off and is expected to remain closed over the weekend, the force added.