I haven't read most of the other responses, but I did read some, and your "what happened" post. If she moves closer, and you are able to take the kids overnight, please never do it without their mother's knowledge! Just a few of the ways that would be a bad idea: you'd be dishonest; you'd be teaching the kids to be dishonest; you'd be putting the idea in the kids' heads that their parents don't care about them; the parents would be unable to find the kids in an emergency; and most seriously, taking children from their home without permission of the parents could result in kidnapping charges against YOU!
Keep in mind that one of the ways child molesters set kids up is by telling them that it's OK to keep secrets from parents, and often they choose frightened or troubled kids and tell them, "I love you so much more than your parents do." If you teach these kids that it's OK to sneak out of their house, and spend the night somewhere without telling their parents, the next time someone ELSE suggests that to them, it won't seem odd or inappropriate. They will assume that the other situation is safe, because you would have taught them that going with someone in secret is safer than being at home. It's a crazy world out there, and even if we mean well, we shouldn't do something that might backfire.
Back to the original question, the reason that CPS doesn't put a number on how old kids must be to be alone or in charge of a sibling is that it depends entirely on the child or children in question. Both growing up and now as a parent, I've seen responsible 7 and 8 year olds who could be left in charge of siblings without an issue, and I've seen much older kids who figured that the absence of adults was an invitation to break every rule and engage in risky behavior. Cub Scout and Girl Scout books start covering babysitting topics with kids as young as 7, so they'll be prepared when and if they're needed. I started babysitting for other families (I'm the youngest child in mine) when I was 11. I've hired 11 year old sitters for my kids. I've also had 15 and 16 year old sitters that I never asked back (one left a baby in the same diaper the whole time, so she had a bleeding rash by the time I got home).
Don't worry too much about things like wild dogs. Yes, we need to be aware and cautious about any wild animal, even chipmunks or squirrels, but coyotes and the like are far more likely to run away than to attack a child. Anything they consider too big and healthy to be easy prey, they leave alone. If a family lived in the woods, you'd be wasting energy if you worried that bears or mountain lions would get into the house and attack them at night; this is the same type of situation. It sounds as if they're reasonably aware and cautious when they need to be.
It's good that you're trying to be a good friend, and a protector of children, but we all need to be very careful not to assume that all parents need to do everything the same way. No two families are the same. Even within a family, what worked with one child may not work for another. Doing things differently is OK.