I think more details are required.
Does she do this often or was it a one time thing?
Why did she give it to her?
Had the daughter been sick, coughing all night, and having trouble sleeping?
Did she adjust the dosage based on the girl's size?
Whose medicine was it and how did she get it?
Without more info, I don't really think anyone can give you a real answer. Do I think what she did is smart? No. Do I think it's illegal? Probably not if it was a one time thing, especially if there was a somewhat legitimate reason for doing it and the dosage was appropriate.
I certainly wouldn't go to the police with it unless it's an ongoing thing or if she's doing it simply to knock the kid out so she doesn't have to deal with her.
I definitely think you need to talk to the mother and express your serious concern over the action.