I wouldn't leave children of those ages alone, I think it might actually be more of a risk when there's two of them (if mine are anything to go by, much bigger age gap but far more likely to do something daft)!
It's unlikely anything would happen but things do happen. I was left alone from around the age of eight, I was a very sensible child and we lived in accommodation attached to our business so there was always someone around. Due to layout, it would probably be about the same as having a next door neighbour home and aware that there was a child alone.
However, as sensible as I was, I still managed to accidentally start a fire in our lounge when I was 10. Did I dial 999? No. Did I get out of there as fast as possible? No. In panic, I thought my mum would go spare and tried to put it out myself. Everything i'd been taught was forgotten in an instant.
I did manage to put it out but it was a bloody stupid thing to do. My mum, of course, wasn't angry and gave me a big hug saying that it didn't matter as long as I was ok. It really shook her up though.
As an adult, I can't guarantee that i'd remain calm in an emergency despite knowing exactly what to do, so I dont think i'd be trusting a child to do the sensible thing either.
Just my experience though so each to their own I guess 