This happened to me as well. During the Nanny Knows Best quest you pick the baby up. When you whistle your sim holding the baby to places they will go with them. So you can have the baby physically with mom. You can delete mom and make a new mom so the option to adopt comes up. This will remove the gold ring baby bonus if the baby has that tho.
You can make do with the annoying map of baby being alone in a house and the technical owner and wait until they age up to teen so they can socially ask to move in and fix it technically at that point!