Provincial police say a man, a woman and a child were taken to hospital to be treated for smoke inhalation and shock after Thursday's fire at their home on EloiLachapelle Street around 3:30 p.m. ET.
The house was destroyed.
It's too early to know if the missing children died in the fire, but the search among the house's debris resumes Friday morning.
The evening before, Gracefield Mayor Joanne Poulin was among members of the small community who gathered near the scene of the fire.
"It's tragic," Poulin said. "For a small community like us, it's not easy for everybody."
Gracefield is about 90 kilometres north of downtown Ottawa.
Correction : An earlier version of this story gave the boy's age as two and the girl's age as four. In fact, the boy is four and the girl is two. (Feb 27, 2015 7:57 AM)