"But if someone wants to go to SkyCity and leave their children locked in the car ... then not only is it that totally irresponsible and no doubt in breach of the law, but that can happen in an environment from a supermarket through to a casino,'' he told TV One's Breakfast show.
"In terms of harm minimisation, which is what we worry about, casinos are arguably a better environment.''
Mr Key said that unless all casino licenses in New Zealand were ripped up, such incidents were always a risk.
"The evidence seems to support that in a casino they're in a better environment than say, attached to maybe a pub deliberately targeting low-income people.''
Mr Key said SkyCity may well get more pokie machines as part of the deal, but the overall number of machines would still fall.
"Overall we have a sinking lid policy on them, so we're actually effectively closing down pokie machines in other parts of Auckland and around the country.
"So even if this deal is to go through, the number of pokie machines in New Zealand would actually be falling. They just wouldn't fall quite as rapidly.''
The proposed convention centre would generate 1000 jobs to build and 900 to run, as well as bringing a lot of people to New Zealand, Mr Key said.
"I reckon that's not a bad deal for New Zealand.''
The children in the most recent incident were taken into Child Youth and Family care.
SkyCity general counsel Peter Treacy last week described the incident as the worst of its kind in the 16 years the casino had been operating.
Since 2010, 231 children have been left alone by their parents at casinos around the country. Internal Affairs spokesman Trevor Henry said that in 2010, 84 children were found alone at casinos in 51 incidents. Of those, 11 were in car parks.
Last year, there were 59 incidents involving 101 children, with 14 being found in car parks.
Already this year there have been 25 incidents involving 46 children, including six in car parks. Most cases were in Auckland, Mr Henry said.
