The two dead children were aged six and nine months.
Prosecutor Christian Du Four said the attacker, a 20-year-old man, rode his bike to the Fabeltjesland day care center about 10 a.m., found it unlocked and went in."After he entered he started slashing at everyone he ran into ... the day care workers, the children," Du Four told a news conference.
He said the man wielded a nearly 8-inch knife and had painted his face white with black patches around the eyes. He is a 20-year-old Belgian, not of foreign origin," he said.
One worker tried but failed to disarm the intruder and another female worker was stabbed to death, Du Four said. After the attack, the attacker walked out and got back on his bicycle. He was arrested in a nearby supermarket shortly afterward.
Eleven children and two female staff were being treated at local hospitals, authorities said. The assailant arrived at the day care center in Dendermonde, 30 km (20 miles) west of Brussels, at around 10 a.m.
Contrary to Belgian media reports, the man was not a psychiatric patient, Du Four said.
The suspect was formally arrested last night by examining magistrate Christian Dufour.