Other children have died in unlicensed daycares, including a baby who was shaken to death in 2011 and a toddler who was found at the bottom of a pool in 2010, said NDP critic Monique Taylor.
It's not yet known what caused the two-year-old's death Monday at the daycare in Vaughan.
But it's the government's job to make sure that people have safe places for their kids while they're at work, Taylor said.
"It's quite obvious that we have children dying, and that we need to make sure that the ministry is doing their job," she said.
"We just need to make sure we get it right."
Education officials have admitted that they failed to follow up on two of three complaints lodged against the Vaughan daycare. But they say they're taking steps to investigate the facility and look at other complaints filed against unlicensed daycares.
They'll determine whether there are grounds to lay charges under the Day Nurseries Act, which could result in a year in jail and fines of $2,000 a day, officials said.
They're also reviewing all complaints filed against unlicensed daycares in Ontario in the past year to see if the ministry followed up. It will be completed within a week, officials said.
"This is a tragedy that no parent should have to experience and my heart goes out to the loved ones of this child," Education Minister Liz Sandals said in a statement late Thursday.
She said she learned Thursday that two complaints lodged against the daycare went unanswered.
"This is clearly unacceptable," she said.
Complaints were made about the number of children at the daycare in October, November and December 2012. But the ministry only investigated the one in November with a visit to the daycare, ordering that it comply with regulations.
"This was a clear non-compliance with ministry policy," Sandals' office said in the release.
"It has been a longstanding policy to conduct an initial site visit within five business days for all complaints regarding unlicensed child care providers who may be caring for more than five children under the age of 10."
Officials will review the lack of compliance and "take appropriate action as necessary," her office said.
The ministry will respond to any outstanding complaints immediately, it said.
But the NDP say that's not enough.
"Well, the ministry didn't do the job right the first time, so we need to make sure we get it right," Taylor said.
Ombudsman Andre Marin said he's directed his staff to see if an investigation is warranted, "in light of the serious allegations raised in Ms. Taylor's complaint, the strong public interest in the issue and the potential systemic implications."
The ministry's investigation of the Vaughan daycare will include interviewing parents to determine how many children were under its care, the ministry said.
Unlicensed providers can legally care for no more than five children under the age of 10 — in addition to their own children — but there were reports that there were far more kids at the daycare when the coroner arrived Monday.
It was shut down on Tuesday due to unsanitary conditions.
The ministry only conducts inspections of unlicensed daycares if a complaint is made, said Lauren Ramey, a spokeswoman for Sandals.
It receives 200 to 300 complaints each year and have 54 program advisors carry out the work province-wide, she said.
Progressive Conservative Leader Tim Hudak said he also wants answers about whether this was an isolated incident.
"This seems like it was the government that dropped the ball," he said.
The Home Child Care Association of Ontario, which represents licensed home daycare agencies, said the governing Liberals should require all childcare providers to meet minimum licensing standards.
Under the current rules, licensed daycare providers can't care for more than two children under the age of two and no more than three children under the age of three, it said.
But Taylor said the province shouldn't shut down unlicensed daycares, which some parents say are the only ones they can afford or their only option when other facilities are full.
"We need to ensure that what we do have in place is being done safely, and that our children are coming home at the end of the day."
Note to readers: This is a corrected story. An earlier version incorrectly said that Sandals learned that the ministry failed to respond to two complaints on Tuesday. In fact, she said she learned of it Thursday.