If you've ever heard: but WHY did he do this? What was DRIVING your subject? What INSPIRED her to...
My friend, you are in need of establishing motivation.
Motivation is the heart of your subject's actions. It's a key element that your reader can connect to and be inspired by. It takes a biography from "telling" to "showing."
This doesn't mean you need to psycho-analyze your subject; the reader just needs to see the connection between how your subject grew up to who s/he became and what s/he did. Because the picture book audience is intended for children, typically that is set up through the childhood lens, because it allows the audience to connect on a relatable level.
How or what in your subject's childhood helped him/her solve the problem s/he faces later in life, that led him/her to do the thing (s) you are writing about?
Here are some examples (I linked to these so you can see the previews):
1. Funny Bones by Duncan Tonatiuh
Here, you can see the progression nicely; the author sets up why Posada become a printer & why he was into art, and leads into, why did he start doing political cartoons (you can't see in the preview, but it explains the political atmosphere in Mexico, and how most people couldn't read and write, thus Posada was doing the cartoons to help spread the word and reach people).
2. I Dissent by Debbie Levy (Author) and Elizabeth Baddeley (Illustrator)
Here, you can see who inspired Ruth (her mother), the atmosphere she grew up in, and how her personality built into who she became.
3. For the Right to Learn by Rebecca Langston-George (Author) and Janna Bock (Illustrator)
Here again, you can see how Malala developed her love and passion for learning in childhood, and why it was important to her, and when that was threatened to be taken away from her, she fought back.
4. A Picture Book of Martin Luther King, Jr. by David A. Adler (Author) and Robert Casilla (Illustrator)
Here you can see where Martin Luther King Jr. got his peaceful side (father was a pastor, attended church) while establishing the turmoil/problem within the atmosphere he lived in, both of which help the reader to understand why he approached the problem the way he did, what the problem was, and why he was passionate about advocating for equal rights.
It isn't so much a matter of picking THE defining moment in your subject's life so much as WHAT about your subject's life led him/her to who he/she became.
Within the span of a few pages and a few hundred words, of course!