Pennsylvania mom pleads guilty in 3-year-old Scotty McMillan's murder

WEST CHESTER, Pennsylvania -- A mother has pleaded guilty to third-degree murder in the beating death of her young son in Pennsylvania and agreed to testify against her former boyfriend in his capital murder trial.

Thirty-three-year-old Jillian Tait also pleaded guilty Wednesday to conspiracy to commit first-degree murder and child endangerment in the November 2014 death of 3-year-old Scotty McMillan in Coatesville.

Prosecutors said the plea agreement will spare Tait a possible death sentence and she will be a "critical witness" in the first-degree murder trial of 25-year-old Gary Lee Fellenbaum III.

Authorities allege that the boy was hung upside down and beaten with fists, metal rods, whips and a frying pan, and the couple went shopping and ate pizza as he lay dying after weeks of escalating abuse.