Sergeant Christopher Fletcher and his colleagues smashed their way into a semi-detached house in Spalding after concerns were raised about the welfare of Katie Edwards and her mother, 49-year-old Elizabeth.
As he and another officer went upstairs they found the blood-covered and partially clothed body of local dinner lady Ms Edwards.
Searching the house in Dawson Avenue for the victim's daughter, the Lincolnshire Police officer then went into another room where there was a child's bed.
He "pulled" at a quilt on top of the mattress but it felt heavy, Sgt Fletcher recalled.
Her toys were laid out nearby, he added.
The jury was told that "severe force" was inflicted on Katie, pathologist Dr Frances Hollingbury concluding that she died as a combination of loss of blood and smothering.
Peter Joyce QC, prosecuting, said an artery had been damaged and a stab wound had penetrated a bone on the right side of her spine.
Dr Hollingbury said that Katie's mum Elizabeth, whose body was found in an adjoining bedroom, had suffered two stab wounds to her neck, cutting her jugular veins. She also had a stab wound to the shoulder and cuts to her hands caused when she defended herself.
A 15-year-old girl denies the murders of Liz Edwards (49) and her daughter Katie between April 12 and 15 this year. A 15-year-old boy has pleaded guilty to murder and will be sentenced at the end of the trial, which is expected to last up to eight days.