Go to the Ember Island menu. Select "Enter Code", then enter one of the following passwords to have that character appear at the Volleyball Theater.

Appa (Chibi)
Enter Momo, Sokka, Toph as a password.

Azula (Chibi)
Enter Pakku, Iroh, Appa as a password.
sroea

Momo (Chibi)
Enter Aang, Katara, Zuko as a password.