Irony is a big theme in the story. Irony is the opposite of what is expected. You have a young woman in the 1800's that is married; during this time period, women were raised for the sole purpose of marriage. They weren't raised to find fulfillment in a career, in hobbies, or in any other way than through marriage. Marriage was the end-all of their existence, and supposed to be what ultimately fulfilled their every happiness. The irony in this story is that she was not happy, and she was even elated at her husband's death. This is unexpected, especially since "she had loved him-sometimes" and he had never harmed her, who had "never looked save with love upon her". Another irony is the fact that her husband was indeed not dead, and then the last and final tragic irony is her death. The doctors claimed it was "of joy that kills" but we know better. She was shocked and dismayed to see her "freedom" taken from her as her husband walked in the door.
This theme of irony works well with other major themes such as identity, self-fulfillment, and roles in marriage. I've provided links below that discuss those themes in more depth. Good luck!
Besides consulting enotes for information about The Children's Hour's themes, in summary, this powerful play is a tragedy about the power of a lie.
Simple words can destruct a human being.
Although Hellman claims that her character Mary, the child that spreads a lie and bullys the other students to perpetrate the lie, is not evil, others may, indeed see her as exactly that.
Another theme is that of defining a victim. And the moral dilemma of justifying how to judge a victim that becomes a victimizer.
Hope this helps. The link below has info that can be of great help.