The little girl who plummeted to her death from the window of her family’s Bronx apartment had just turned 4-years-old — and was supposed to have a birthday party Sunday, her heartbroken mom said.

Helena Flores celebrated her birthday on Feb. 5, and was looking forward to a Peppa Pig-themed party — her favorite — when tragedy struck just before 5 p.m. Saturday, mom Hilda Flores told The Post via Facebook.

“Because the kids room was so small and had two radiators in it, we would sometimes leave the kids top window open just about two inches,” Flores wrote. “Our beloved 4 year old Helena found a way to [climb] up, open the window wider and tragically fell out our 4th floor apartment [sic].”

The tot tumbled to East 140th Street below, and was pronounced dead at Lincoln Hospital.

“She was our little diva our little princess. A small happy innocent child,” Flores wrote. “We are devastated. Our little girl is gone.”

Flores — who said that her daughter “had autism and was unaware of [the] danger” — said that she and her family can only take solace in knowing that Helena died quickly.

“We was told she died on impact,” she wrote. “Although no amount of words could console my family and I, knowing she didn’t suffer gives an inch of peace.”

Police are investigating the death, but have not charged anyone.

Flores insisted that her family took great care to keep the Mott Haven apartment safe for Helena.

“We childproof the whole house, we made all of our children feel safe and loved,” she wrote. “One mistake changed all of this in a split second.”

She also urged other families to learn from their loss: “Hold on to your children cause accidents like this can happen to anyone, at any moment.”

Additional reporting by Aaron Feis