I don't know much about the fourth season of the movie, Water Prince. As a fan of online literature, I have a lot of knowledge, but not everything is up to date. If you have any information about other movies or TV series, please let me know. I'll try my best to answer your questions.
In the second book of the Nirvana in Fire Board, Prince Yu's child was Xiao Pingzhang. Xiao Pingzhang was the foster son of Xiao Tingsheng, the King of Changlin. He led the Changlin Army on the battlefield and became a pillar of the imperial court. Xiao Pingzhang was adopted by Xiao Tingsheng, who was one of the children Mei Changsu had rescued from the Yeyou Court. Therefore, Xiao Pingzhang was Prince Yu's child in the second movie.