How do animals know who their child is?
Penguins (and many birds) recognize their offspring and mate thanks to vocalizations.
It's called individual vocal recognition, it's like when you recognize your friend voice among a group of chatting people.
Scientific article references for penguins: 
http://www.sciencedirect.com/science/article/pii/S0003347299910862
http://rspb.royalsocietypublishing.org/content/267/1448/1081.short