Police were called to the home at 25 Granite Ridge Dr. in the community of Stittsville at about 5:30 p.m. ET, according to a media release.
The father of the children found the bodies, police sources told CBC News, and was later questioned by police.
Investigators suspect the mother — identified by acquaintances as Alison Easton, though police sources say she also sometimes went by the name Alison Corchis — was responsible for the deaths of the couple's son Alex, 10, and daughter Katie, 6.
A sharp weapon was used, but police sources would not say what type. It was a "disturbing scene," they added.
A neighbour, Jeff Wilk, said the woman living at 25 Granite Ridge was a Neighbourhood Watch co-ordinator. She introduced herself to Wilk's family when they moved into the neighbourhood more than a year ago, and gave them a welcome package.
Daughter's 6th birthday last weekend
She was often seen walking her children to and from nearby Stittsville Public School on Granite Ridge Drive, Wilk said by phone.
Another neighbour, Tonya Marczak, said her daughter was Katie's best friend. They celebrated Katie's sixth birthday together last weekend.
"They seemed like such nice people. To hear that there's three dead, it's just devastating," Marczak said. "I don't undertsand how this happens, I don't, especially when I've got little kids of my own."
Marczak said she told her children there was a fire at the home, and nothing else. She said her daughter was best friends with Katy, who had just celebrated her birthday with a fancy dress-up tea party last weekend.
"I honestly don't know what to tell them at this point," she said. "Do I send them to school tomorrow, do I not? I have no idea."
The two children went to school across the street. Katie was in kindergarten; Alex was in Grade 5. Neighbours said their mother stayed at home.
The major crimes unit is handling the investigation.
No further details were released.