The Child (and no, I'm not talking about Baby Yoda) is very energetic, innocent, and childlike. They are never very serious about any situation, like a child, and their lines are relatively short, and one of the characters that you really need to watch gameplay for full context. Their voice should be high-pitched, and sound like...well...a child! With that description in mind, try listening to Hestu's voice from Legend of Zelda, Hyrule Warriors. I do recommend, however, to tone down the goofiness in his voice. I encourage being fun with the voice, but not so much so that the audience doesn't take them seriously at ALL. 

https://youtu.be/jQxJMFXtMpg