Why are Princess Iron Fan and Niu Wangmo not afraid of monkeys and Nezha?

Because Nezha has Gankun Circle, which can subdue cattle, and cattle are afraid of Nezha. Princess Tiefan was afraid that Niu Wangmo would be killed by Gankun Circle, so she gave in.

Princess Iron Fan, Niu Wangmo, the Monkey King and Nezha are all characters in The Journey to the West.

The Journey to the West, one of the classical Four Great Classical Novels in China, is the first romance novel of Han nationality in ancient China, which was perfected by the novelist Wu Cheng'en in Ming Dynasty. Mainly describes the story of the Monkey King, Pig Bajie and Friar Sand protecting Tang Priest's Buddhist scriptures. They met with eighty-one difficulties along the way, turned the devil all the way, saved the day, and finally arrived in the Western Heaven to learn from the scriptures. According to Tang Sanzang's Buddhist scriptures and Han folklore.