Hi,
I have a friend who told me she did this recently and I was quite shocked.
I think it was for a couple of hours, so not all day, but the other dc was supposed to be in her care, no? It sounds like this was the first time this dc had been to the house.
I'm sure it's quite common for people to leave their 11 year olds home alone occasionally, but if my dc went to a friends house at that age, I'd expect an adult to be in the house.
What does anyone else think?