I just saw this for the first time now too. I haven't noticed a sim abandon a toddler in my games. I suspect the toddler would have been kicked out of town at 3 am or 5 am if it hadn't been rescued.
This makes me wonder if something similar happened to a townie family in Barnacle Bay. In my Big Game I had a sim, I think it was Bart Inkbeard but not sure. They were at boarding school that I play and when I returned him to his family I found an extra child living with them. They were all fair skinned with blonde hair and the extra child was very dark with blond hair. I took her to the boarding school so I could control her and investigate. She was the only sim in her family tree. Her family name was different. I have no idea where she came from. The household she was living with didn't really have room for her so I've kept her at the boarding school and played with her ever since. She just became a teen in the most recent episode of Oscar's Childhood - or the first episode of Oscar's Teen Years because he aged up in the same episode.
