Simple; Descriptive
Felice Holman uses a simple, straightforward writing style to ensure that we fully understand Alex's story, while simultaneously using descriptive imagery to paint a broader picture of Soviet society.
Don't worry, though; you're not going to find anything too fancy in The Wild Children. Holman places a high premium on clarity and doesn't see the need to go crazy in order to get her points across. Instead she focuses on describing Alex's emotional reactions to the events of the novel with a great deal of clarity. This allows us to focus on the actual story being told, rather than wasting precious brainpower trying to decipher five-dollar words and complex sentences structures.
That said, Holman fleshes out her story by using the book's setting to illustrate broader points. We see this a lot when Alex is staying is Moscow. For example, Holman describes how an "enormous square seemed almost empty except for […] a couple of carts with wooden wheels being drawn, not by horses, but by peasants" (3.17) to illustrate the flailing status of this once-great city. By combining these vivid descriptions of the Soviet Union with her straightforward approach to character development and plotting, Holman makes us feel like we're actually there, not just reading a book.