Social workers and police knew about the violent history of two men but did not intervene before their children were killed, in separate cases in the one UK county.

Police found evidence that two-year-old Dylan Tiffin-Brown was being left alone in his drug-dealer father's home, where he was exposed to dangerous narcotics.

Dylan Tiffin-Brown, 2, was murdered by his father. (Northamptonshire Police)

Despite that, social workers did not intervene, deeming him not at risk, a review has found.

Kennedy had a "chronic history of domestic abuse and extensive history with the police for drug-related offences", the review said.

Two months later, Dylan's father Raphael Kennedy, 31, beat him to death in a horrific attack.

Dylan suffered 39 injuries to his face, neck, torso, arms and legs.

A post-mortem found traces of cocaine, heroin and marijuana in his system.

Kennedy was sentenced to life in prison for the attack.

Five months later, in April 2018, one-year-old Evelyn-Rose Muggleton was beaten to death by her mother's partner Ryan Coleman, who was caring for her.

Evelyn-Rose Muggleton was beaten to death by her mother's partner. (Northamptonshire Police)

Evelyn-Rose suffered traumatic brain injuries in the beating.

The National Health Service and school staff had recorded signs that she and her four siblings were being neglected, but did not pass on their concerns.

Meanwhile, social workers concluded that her mother was "parenting well", a review found.

Coleman was also sentenced to life in prison.

Both cases the responsibility of the child protection services office at the Northhamptonshire county council.

Opposition members have referenced the reviews in calling for council leader Matt Golby to resign.