If your kids are responsible, that's the first important thing.
The second important thing is how much independence have they had to this point in time? Are they used to, say, walking/biking to a friend's house on their own, pulling together lunches/snacks, and that sort of thing? Are there neighbors/friends around who are available in a crisis?
The reality is that kids have been on their own or under minimal supervision since time immemorial. Is it the best option? Nope. But is it something you can get through as a family for a short period in a true emergency situation? Absolutely.
FWIW, I was a latchkey kid from the age of 7. Didn't have a choice -- my divorced mom needed to work and get her degree, she didn't have the money to hire help, and this predated the current mass of daycares around. It wasn't my favorite thing, but I was fine; I was responsible and fairly independent and self-sufficient, so I was the right "fit" to be in that position.
But I also had had a lot of independence before that time. From about 4, my friends and I used to run through the neighborhoods on our own, because that's what you did then. I learned how to cook around 5, when my mom was in a very bad traffic accident and was bedridden for over a month; it wasn't fancy, but I learned how to open a can of Franco-American, empty it into a pot on the stove, heat it up, dish it out, and clean the dishes -- even though I was still short enough that I had to climb onto the countertop to do half of that.*
So if your kids are responsible and capable of feedling themselves and used to having a reasonable amount of independence, then I think you're completely fine to give it a shot and see how they manage -- perhaps with some incentives for good behavior all around. OTOH, if they've never stayed on their own for a couple of hours or played unsupervised or gotten themselves to a friend's house, then you might be expecting too much from them all at once.
*Also had to walk 9 miles to school uphill both ways through the snow. ;-)