Lizabeth is acting childish because she is trying to destroy Miss Lottie's flowers and annoy her just for fun.
Lizabeth woke up to her parents talking, and she hears her father crying. Her father lost his job. The father is now weak and her mother is the "rock" in the family. Her family has switched roles and is falling apart.
Lizabeth has had enough and goes to destroy the flowers. She manages to destroy them but Miss Lottie finds her. Lizabeth feels really bad after and learns that you can't have compassion and innocence and she needs to change like her family, which is growing up.