Twenty-three-year-old Michael Rios pleaded guilty Thursday to shaking to death the 13-month-old baby of his former girlfriend in May 2013, bringing small satisfaction to the grieving members of baby Madison Alexa Reid’s family.

Rios, who pleaded guilty to first-degree manslaughter, will be sentenced Sept. 29 to up to 14 years in prison. He accepted a plea offer involving a sentence of 20 years in prison, suspended after 14 years served, with five years of probation. Defense attorney Christopher L. Morano will have the opportunity to argue for a shorter sentence, though Judge Hillary B. Strackbein indicated she is not making any promises due to the seriousness of the crime.

The baby’s family members, many of whom have been wearing hot pink T-shirts that read “Madison Matters” to Rios’ court appearance, say the sentence is too lenient but that they derived some satisfaction from hearing Rios admit his guilt.

“Even if he serves the full 14 years, he’ll be 37 years old when he gets out,” said Colleen Reid, the grandmother.

Rios had been dating Madison’s mother, Elizabeth Reid, for a few months and had agreed to take care of the toddler and drop her off at day care after Reid left for work on May 2, 2013, according to Norwich police. The child was unconscious and bleeding from the brain when he took her that day to The William W. Backus Hospital. She was flown to the Connecticut Children’s Medical Center, where she underwent emergency surgery, but she succumbed to her injuries during the early-morning hours of May 3.

Prosecutor Theresa Anne Ferryman said Madison suffered a subdural hematoma and retinal hemorrhaging, and the state Office of the Chief Medical Examiner ruled her death a homicide.

Rios told police that Madison became unresponsive after falling from a bed and he “shook her very hard” to get her to respond.

Rios has been free on $100,000 bond while his case is pending. Madison’ step-grandfather, Rhode Island dentist Lawrence E. Berkman, asked the judge to lock up Rios between today and his sentencing date.

“He should not be allowed to walk free for six to eight weeks while we visit a grave,” Berkman said.

Strackbein did not order Rios locked up but told him he would be subject to the entire 20 year sentence if he gets in trouble before his sentencing date or fails to show up.

k.florin@theday.com