The mother of the 3-year-old blamed for last month’s Bronx blaze that killed 13 people was “known to’’ authorities and child-welfare workers for not watching the boy — yet the city did nothing, new court documents allege.

Eleven claimants representing four people killed and eight more injured in the horrific fire Dec. 28 filed a notice to sue the city for more than $110 million Thursday.

The victims and their representatives allege that the mother of the toddler who started the blaze was “a person known to authorities and to the [city] Administration for Child[ren’s] Services Department for not watching and taking care of her child.

“The claimants contend that had said Administration for Child Services Department and/or other City agencies followed up on complaints and taken the child away from the mother, as they should have, said fire would not have occurred,” the court papers say.

Authorities said the 3-year-old boy’s mom had left him and his 2-year-old brother to watch TV while she went to take a shower.

The 3-year-old wandered into the kitchen and began playing with the knobs on the kitchen stove, sparking the deadly blaze. His mother allegedly later told cops that it wasn’t the first time he had played with the stove.

The mom fled her burning first-floor apartment with her two boys — leaving the door open, which allowed the blaze to spread quickly to the top of the five-story, 26-unit building.

The notice of claim takes aim at the city, FDNY, city Department of Housing and ACS, documents show. Each of the claimants is demanding at least $10 million.

The pending suit will also charge that the city failed to ensure that the building had working smoke detectors — even after issuing violations to the landlord for having faulty equipment.

“The City through its agencies needs to do more to ensure that landlords operate buildings safely and within code so as to prevent further tragedies like this one,” said lawyer Robert Vilensky, who is representing the victims.

“If the City issues a violation and doesn’t enforce the violation by making a landlord comply, the violation then is useless and horrific events like this one occur.”

The suit adds that the city didn’t properly maintain a nearby fire hydrant that was frozen when firefighters arrived at the scene.

According to the court papers, the frozen hydrant caused first responders to lose “precious time by having to look for and find fire hydrants.”

The victims allege the city allowed “water to build up in the fire hydrants thereby allowing the line to freeze” and that cracks in the hydrant “allowed cold air in and caused the fire hydrant lines to freeze.”

“Fire hydrants need to be continuously monitored and checked to prevent freezing because in a raging fire, every second counts,” Vilensky said.

The FDNY said it cannot comment on pending litigation.

A spokesperson for the city Law Department called the fire a “tragedy for New York City and its residents.”

“We take these allegations very seriously and will review the notice of claim once we receive it,” the rep said.

Additional reporting by Yoav Gonen and Danika Fears