I don't think all 9 year olds are responsible enough to be left in charge of a 6 year old. My mother, having 2 younger children at that time, made me walk to school on my own at 5, and I was bullied. I remember feeling very 'inconvenient' when my father had to walk with me a few times before the bullies gave up.
The following year, my sister started and I was only 6, she was 5. I was so frightened at having to be in charge of her. It really wasn't fair, I could see other women walking into school with their families and wondered why my own mother wouldn't.
I didn't allow my own DCs out on their own until they were 9, as a result, and they were certainly never allowed to be in charge of anyone else. A friend tried to make DD aged 8 be in charge of her 2 year old when they were both out in her garden once. While DD was up a tree, 2 year old fell in the pond. As DD was an incredibly sensible child, I could see how she would think it possible to trust her, but children of that age have the attention span of a gnat.
The woman's OH had refused to cover his pond, but he did give in after that incident.