The song is called "Written to Father" and was sung by Lu Huanyu.

Lyrics: I always get into trouble but you never stop me/Because you don’t want to stop the joy of growing up/My son has grown up/Let me dare to act like a man/Let me carry the responsibility on my shoulders/Other people’s family Children study hard and make progress every day. They are obedient and they are role models for children all over the world/I am different from them/I am Nezha/I was born a nine-foot-tall Barbie King Kong/I also know that you have doubted, hesitated, and Mom quarreled, but you never gave up. You said forget it and just gave birth to a stronger daughter/But I am a man/I put the responsibility on my shoulders/I watched you, day by day/I learned, Bit by bit/every move I make has your mark/it’s you who taught me true courage/it’s you who taught me to stand upright——