Mulkey said her son's death was unimaginable - and inexplicable.
He had no broken bones, she said. No bruises. No signs or symptoms of abuse.
"They said I might never know what happened," she told the Post.
That's why Mulkey said "it came as a shock" when investigators told her that Haddix's babysitter, 43-year-old Lori Conley, had been arrested last week on charges of child endangerment and murder in the infant's death.
Haddix, it seems, fell asleep at his babysitter's house one afternoon last month and never woke up.
Conley told police investigators that the child had been fussy while in her care, so she gave him an adult dose of the common over-the-counter allergy medicine to put him to sleep, according to the Columbus Dispatch.
Authorities said the Benadryl proved fatal.
Conley, who is not a licensed day-care worker, was caring for eight infants on May 13 at a residence in Reynoldsburg, Ohio, not far from Columbus, according to the Dispatch.
She told police that she gave Haddix two Benadryl tablets, according to the probable cause statement. The Columbus Dispatch reported that she said that when she went to check on him in a back bedroom, she found him unresponsive in a bouncy seat.
She said she grabbed the boy and ran outside, where a neighbour rang emergency services.
Authorities said in a statement that police were called about an unresponsive child; first-responders transported Haddix to a nearby children's hospital, where he was pronounced dead.
"Our beautiful angel Haddix James Mulkey was unexpectedly called home to be with the Lord on May 13, 2016," his family wrote in an obituary. "In the 8 months we had him, our lives will forever be changed from the joy and love he provided."
His mother has said she wants Haddix's death to serve as a warning to other parents about the dangers of over-the-counter drugs.
"This is a common practice that people think it's okay to give your babies medicine . . . to help them sleep," Mulkey told reporters last week, according to the Columbus Dispatch. "This didn't have to happen."
"I will always have a broken heart because a piece of me is gone," she added, according to the newspaper. "We had so many hopes and dreams for him and now it's all gone."
One to two tablets is the recommended adult dose for Benadryl, or diphenhydramine, an antihistamine that can cause drowsiness. The medication label warns against giving the adult form of Benadryl to children younger than 6.
Even the instructions for Children's Benadryl advise consumers not to administer the drug to children younger than 2. The label warns against using the children's version of the drug "to make a child sleepy".
Heeding warnings from the Food and Drug Administration on cough and cold products for toddlers, drug manufacturers voluntarily pulled such products for children younger than 2 from pharmacy shelves in 2008.
In addition, manufacturers have re-labelled cough and cold products to state "do not use in children under 4 years of age," according to a 2016 statement from the FDA.
"These safety concerns revealed that there were many reports of harm, and even death, to children who used these products," according to the FDA statement. "These reports of harm occurred when the child received too much medication such as in cases as accidental ingestion, unintentional overdose, or after a medication dosing error. In those reports of harm that lead to a child's death, most of those children were under two years of age."
Henry Spiller, director of the Central Ohio Poison Centre based at Nationwide Children's Hospital in Columbus, said that overdoses in children are not uncommon.
"We've seen a number of fatalities," he told the Post.
In 2015, Spiller said, poison centres across the country received nearly 19,000 calls about children younger than 5 who had taken diphenhydramine, better known by the brand name Benadryl. In many cases, he said, parents called concerned about a "double dose" in which one parent gave the child some medicine and then another parent gave it to the child again.
It's unclear how many of those cases resulted in serious medical problems.
Spiller said experts do not recommend giving cough or cold medication to children younger than 2 without consulting a pediatrician.
"It's rarely effective or needed because it really doesn't change the cause of the infection, whether it's viral or bacterial," Spiller said.
The FDA recommends alternative treatments:
1 A cool-mist humidifier helps nasal passages shrink and allows easier breathing (do not use warm-mist humidifiers as they can cause nasal passages to swell and make breathing more difficult);
2 Saline nose drops or spray keep nasal passages moist and help avoid stuffiness;
3 Nasal suctioning with a bulb syringe either with or without saline nose drops, works especially well for infants less than a year old. Older children often resist its use;
4 Acetaminophen or ibuprofen can be used to reduce fever, aches and pains. Parents should carefully read and follow the product's instructions for use label;
5 Drinking plenty of liquids will help the child stay well-hydrated.
For small children, Spiller said, too much cough and cold medication can cause severe problems: Cardiac arrhythmias, seizures, comas - even death.
When medication is needed for children younger than 2, Spiller said, parents should consult with a pediatrician to assess whether it will help the child's condition and then then to prescribe the correct dosage.
The evening before Haddix died from an apparent Benadryl overdose, his mother said, he was happy - even eating his first real meal: spaghetti.
Mulkey told the Post that she first thought her child had died from natural causes.
"A lot of kids just go to sleep and don't wake up," she said on Tuesday. "I was willing to accept that."
But once police told her about the toxicology report, she said, "I felt like he died all over again".
"I feel like he can't rest in peace," she said.
"I can't heal until I get closure, and I can't get closure until everything is said and done," Mulkey said of Conley's case.
The babysitter was arrested on charges of child endangerment and murder, according to her case summary.
She was arraigned at the weekend and is being held on US$750,000 bond. She is scheduled to appear in court on June 14.
It's unclear whether Conley has obtained a lawyer.
Now, Mulkey said, she is trying to raise awareness.
"Stop giving this as a sleeping aid when it's not made for that," she said. "Children are dying because people are using it as a sleeping aid."
Mulkey said she will never get over how her son died.
"I'm very hurt," she said, adding: "I'll miss him forever."
