How do you know language is sinking in?
1) My child started to dream in another language! I've caught him talking in his sleep in Chinese and was delighted.
2) He loves bilingual books. He loves seeing the Berenstain Bear and Dr. Seuss books in Chinese. He also cannot get enough of any book that talks in Chinese and loves to repeat the entire story out loud in the car.
3) He talks to all his school friends in Chinese. He used to be shy and cautious but how shouts "zai jian" to anyone he recognizes.