They should have left it at the scene?
They could have found the mother, who was not at the scene, and let her take the toddler. "Taking into custody" implies into jail, although it certainly could mean held in a police car until the mother could be located. However upon second reading, I think the juvenile mentioned may have been in the other part of the duplex and not the child of the guy who shot the policeman. Stories are so poorly written it's difficult to tell.