If you haven't done it already, check out Stranger Things on Netflix. It has some of the best kid performances I've seen in years. I especially love the work of Millie Bobby Brown who plays Eleven. She does such an amazing job of emoting so much in a very subtle believable way. The moment she opens the music box is a perfect example of this. The shoulder movement, breathing in, her brow expressions. It's all so expressive and readable, yet subtle and not over the top.