Yup, those are S's nicknames for his son and daughter, respectively.
Baby Y is going to grow up and become a beach bum (not a bad life, huh?). She likes what we call the "deck chair" pose. She lounges on Appa, mommy or grandma and contentedly surveys the world around her.
Nowadays, big bro loves holding her. What's more, he always wants me to click a picture while she is sprawled on his lap. She must feel really heavy though; she's almost half his weight.