The boys were found stabbed following a fishing trip in Merseyside, while the girl was abducted from the garden of a house in Llandudno, north Wales.
Merseyside police investigating the death of 12-year-old Robert Gee, of Eastham, Wirral, whose body was found early yesterday morning in woodland at a beauty spot at nearby Eastham Rake, found the body of his best friend, 13-year-old Paul Barker, close by in a ditch yesterday afternoon. Neither body had been hidden.
Police said Robert had suffered stab wounds to his chest but did not have any details of Paul's injuries.
Det Supt Geoff Harrison, who is leading the inquiry, said: "There were signs of clothes being disturbed on both bodies but I can't link that with a sexual motive at the moment."
The boys left their homes at lunchtime on Saturday, intending to go fishing in a pond known as The Carpies.
"Both were riding mountain bikes and carrying fishing equipment in a green haversack. They were also carrying rods. The boys went to The Carpies pond, near the M53 motorway," said Det Supt Harrison.
The boys' fathers launched a search for their sons after they failed to return home. At about 6.45pm they met two other boys near Eastham railway station who said they had seen the missing boys earlier in the day.
Police are asking for these two boys and anyone else who may have seen the boys in the area to come forward.
Meanwhile, North Wales police began a murder inquiry after Sophie Hook, 7, was found dead on a beach at a holiday resort yesterday morning.
Police said the girl, who was from Winsford, Cheshire, had been seriously sexually assaulted before being strangled.
She had been visiting her uncle and had been sleeping in a tent in the back garden with her nine-year-old sister. Her body was found at 7.15am on Llandudno beach by a man walking his dog.
Detectives appealed to anyone who saw anything suspicious on the promenade between midnight and 7.15am yesterday to contact them.
Best friends tragedy, page 3