This has kind of happened to me, and yeah, that's basically what happened. I came home to find my housemate's two-year-old home alone and it had kind of clearly been awhile. My housemate was almost an AITA character, she was constantly basically forcing me to babysit. On this occasion, I think what happened was she knew what time I would be home from work and left shortly before that, thinking it would be okay...except I went out with my coworkers after since I didn't expect a scared toddler to be home alone waiting for me.

Anyway, the police dispatcher was basically like "oh yeah that's bad...but you're there now? And you can take care of the child? Okay, since this isn't an emergency, I'm going to transfer you to the child abuse hotline." The child abuse hotline took my complaint in detail, and law enforcement and a social worker did show up...two days later.

That was actually the first of many calls I made to CPS in the short time we lived together. I can assure you, at least in that time and that jurisdiction, they did not care how annoyed I was as long as I was going to do the bare minimum of care and not present and active threat to the kid.

Also that experience was legit upsetting to the point I still think about it a lot and it's been like 18 years. I cannot imagine making an AITA post about it if AITA had existed back then. Maybe a post to a legal sub or something about child abuse or something just trying to figure out what to do, but not AITA.