Now she cares.

A Brooklyn mom charged with murdering her 4-year-old daughter railed yesterday against prosecutors’ plans to have her young son serve as a star witness when the trial opens next month.

“He needs to be left alone. Just leave him alone!” Carlotta Brett-Pierce fumed.

A Brooklyn Supreme Court judge is debating whether the 6-year-old boy, Tymel, will have to testify via closed-circuit TV, or face his mom in person for the first time since the September 2010 death of his sister, Marchella Brett-Pierce.

Brett-Pierce’s outburst came after prosecutor Jacqueline Kagan said Tymel’s testimony would show his mom’s “depravity” and “reckless conduct.”

“That’s ridiculous. She doesn’t have a leg to stand on,” Brett-Pierce ranted, referring to Kagan. “This is my life, and they’re saying I did things I didn’t do.”

Marchella was just 18.9 pounds when her beaten, emaciated body was found at her Bed-Stuy home.

Kagan said Tymell could testify without facing his mom or a jury once the trial opens April 10.