A “mommy blogger” accused of killing her 5-year-old son by poisoning him with lethal doses of salt — and documenting his demise online — was a monster who “seemed to relish in the attention and sympathy she got from having a sick little boy,” prosecutors charged Tuesday.

Lacey Spears burst into tears as her murder trial got under way in Westchester County Court in White Plains and prosecutors accused of her being a “calculated child killer” in little Garnett-Paul’s 2014 death.

“Although her son was special, he was not sick,” Assistant District Attorney Doreen Lloyd said in opening statements.

“He was not sick until she made him sick,” Lloyd said, according to The Journal News.

Lloyd told the jurors — six men and six women — that the 27-year-old mom researched how to kill her boy with salt and calmly “watched and waited” after she poisoned him.

Spears, from Scottsville, Ky., allegedly force-fed him salt through a feeding tube in the bathroom of a Nyack hospital, where he was being treated for seizures.

He died last January from a swollen brain.

Defense attorney Stephen Riebling told the jury that there are no witnesses or evidence proving that the mom, living in Rockland County at the time, committed the crime.

“There is no evidence in this case that legitimately answers the question ‘Why?’ ” he said.

Spears has claimed Garnett-Paul suffered from health problems that made him unable to keep down food and gain weight.

She chronicled his many doctor and hospital visits on Facebook, Twitter, MySpace and a blog.

Prosecutors will present many of the postings, as well as hospital records and Spears’ cellphone activity.

“These [expletives] think I hurt him!!!!!” Lacey texted someone two days after Garnett-Paul died and four days after she was questioned by cops, according to The Journal News.

Spears is charged with manslaughter and second-degree murder.

Defense attorneys have insisted there will be no mention of Munchausen by proxy, a disorder in which caretakers secretly hurt their children for sympathy.

With Post Wires