Nailla probably knew right away it was not her mom, but she also probably didn't hear the woman claim to be her mom. The way she walks a distance away from the woman shows she's not comfortable with the suspect.
Nailla is probably just like any 5yr old just obeying what teacher tells her to do, so all she thought was that teacher told her she was ok to leave with this person.
I've read comments on facebook and news articles of different people asking why Nailla didn't say it wasn't mom. I don't get how any one could try and place any of the responsibility on a 5yr old. All the blame for this falls on the adults in that school who are paid to keep her safe 6 hours a day.