Just jogged my memory by reading an old diary. When I was 14, I got a babysitting job (friend of family friend). Their child was only 3 years old. I met them in a cafe before I looked after their daughter, but they’d literally go for a whole evening out - 6pm-11pm - and leave me in charge. They wouldn’t go far, maybe a mile or two away in a taxi for food and drink. I think they paid me £15. She was awake when they left and I would do bedtime and then watch some TV downstairs or do homework. I text the parents midway through to let them know she was ok.
We’re not talking that long ago, ten years ish. I’m now about to become a mum myself and I can’t believe anyone would leave their child with a child! I’d possibly trust a very reliable, close relative age 16 or over. I was responsible as a teenager, had experience with children and knew what to do in theory in an emergency, but I don’t know how I’d have coped in practice.
YABU - you’re overthinking, that sounds totally fine
YANBU - I agree with you, it’s irresponsible