Petitbois was to return to Massachusetts on Friday. Authorities there planned to charge him with first-degree murder and assault and battery with a dangerous weapon in the fatal stabbing.
It wasn't immediately clear whether Petitbois, 25, might also be charged with forcing the family to flee with him. Authorities had issued Amber Alerts on Wednesday for the children, the oldest of whom is 3.
Petitbois was arrested later Wednesday on a street in Brooklyn, and the woman and four children were found in a nearby park, authorities said.
"Our witnesses have led us to believe that the children and their mother were taken against their will," said Steve O'Connell, a spokesman for the Essex district attorney's office in Massachusetts.
The mother, Louna Eveillard, at first told New York police she wasn't forced to leave, but now authorities don't believe that to be true, New York Police Department chief spokesman Paul Browne said.
Eveillard was released after speaking to police Wednesday.
Police said they did not know where she was Thursday. No telephone number could be found for her in Massachusetts or New York.
The family took a bus from Boston to New York, where authorities traced the group after Eveillard used a welfare card to buy food, Essex prosecutors said. Eveillard had a wound to her hand, but it wasn't clear how she got it, police said.
Police found the slain woman, Greenland Etienne, 33, with stab wounds to her face early Wednesday in Lynn, Mass., about 15 miles north of Boston.
A man who said he witnessed the killing told the Boston Herald the victim was helping Petitbois's children and their mother, who had sought refuge from Petitbois. The witness, Sabinio Stpruex, said Petitbois tried to make Eveillard leave with him and then stabbed her would-be protector as she tried to call 911.
Petitbois and Eveillard, 26, came to the U.S. from Haiti in 1999, according to Essex prosecutors.
