Under either scenario, Joe is guilty.
In addition to crimes relating to obstruction of justice and moving the body, Joe would probably be charged with felony murder. It doesn't matter that it was an accident, or that Tom was the direct cause of the accident.
California's felony murder rule covers accidental deaths occurring during the commission or attempt to commit specified felonies. (Sec. 189, below) While murder generally requires an intent to kill, accidental deaths that occur during specified felonies are punished as first degree murder even if another person actually causes the death. It doesn't matter that the crime was not completed; attempts are covered as well.
The felony murder statute includes the felony known as "lewd acts with children". (Sec. 288, below) That includes willfully touching of any part of a child’s body "with the intent of arousing, appealing to, or gratifying the lust, passions, or sexual desires of that person or the child". If someone dies during the commission of or an attempt to commit that crime, it's first degree murder.
Joe tried to touch Danny, and Danny ran. Danny died while trying to escape, even though it was an accident. Joe is facing 25 years to life in prison, life without parole or death (although the death penalty is unlikely) no matter what happens to Tom, so his decision to take sole responsibility isn't irrational.
Lewd acts:
http://www.search-california-law.com/research/ca/PEN/288./Cal-Penal-Co de-Section-288/text.html
Felony murder:
http://www.search-california-law.com/research/ca/PEN/189./Cal-Penal-Co de-Section-189/text.html
Made you look!