Panda Bear Print Forum Language

When I write things for kids, from a child's point of view, I sometimes get feedback like, "would a child say that?" Sometimes I completely agree, like a five year old wouldn't likely say they were frustrated, they would instead say they were mad. But my 7 year old has a large vocabularly and uses words like "determined" and "upset". So I take a cue from her and put larger words in my writings.
Here's my question. Maybe larger words are used at times because an adult is writing the story and not a child so without thinking, we use bigger words. But sometimes the bigger words fit and changing them would feel like we were "dummying down" the story so kids will understand it. But challenging children and increasing the vocabulary is beneficial for them, if they don't know the word, they'll ask or better yet- look it up.
What do you guys think?