The apparently deranged woman who stabbed an unsuspecting California mom as she changed her baby girl in the bathroom at Macy’s Herald Square should be thrown behind bars for good, the infant’s grandmother told The Post.

“That person should be in jail for the rest of her life,” the 10-month-old’s paternal grandmother said Saturday of Kerri Aherne, the Massachusetts native accused in the disturbing attack.

“I believe she should go to jail because what she did was not right,” the woman, whose son is the baby’s father, added.

The baby was unhurt in the Thursday attack which left her mother with stab wounds in the back, shoulder and arm, authorities said.

“The baby is fine. I’m happy nothing happened to her,” the older woman said.

The victim — who fought back and subdued her assailant — was going off “a mom’s instinct,” a family friend told The Post.

“I’m not surprised she did that,” the friend, Justine Silva, said.

“She works out. She’s pretty fit,” Silva said of the brave mom.

The mom and her family were supposed to leave NYC the day after the attack but are now stuck in the Big Apple with no clear date of return, Silva added, while the grandmother said the couple is expected to attend court Monday for criminal proceedings in the case.

The trip “was a holiday thing,” Silva, a high school girls soccer coach who knows the family through her boyfriend, said of the couple’s trip to New York.

“They just had the baby not too long ago and they were just trying to get [away] on holiday. This is their first trip to New York since they had the baby.”

The baby is the couple’s only child, and concern for the infant was immediate after word of the horrifying incident spread through the family, Silva said.

“I was concerned for her. I was worried that she might’ve been hurt,” Silva said, describing the baby as “really calm” and sweet.

The randomness of the attack made it especially jarring, Silva said, because of how close she is to the family.

“There are so many people in this world and it just happened to be her — someone that I know,” she said. “The family is a very tight-knit family.”

Silva said she briefly connected with the baby’s father after the attack and described him as exhausted.

“He probably was up all night and it was stressful for him,” she said. “That’s his only child. He is an amazing dad.”

“I’m happy that she’s gonna be okay,” Silva added.

Aherne – who was released from a psychiatric facility the morning of the attack – has been charged with attempted murder, assault, child endangerment and weapons possession in the disturbing incident.

Prosecutors said Aherne purchased the knife inside Macy’s before allegedly targeting the unsuspecting mother. 

Aherne, who authorities said has a history of mental illness, allegedly confessed to authorities that she heard voices in her head telling her to kill, prosecutors said.

Online trolls later flooded what appeared to be Aherne’s Facebook page — where she had previously made anti-Trump comments — with vitriolic, politically charged comments after news of the stabbing spread.

“Kerri Aherne we hope everything in the world bad happens to you and only you. Your mom and dad are the pathetic ones here,” one user wrote.

“Your insanity caught up to you. If you hate America, you’ll love prison,” another added.