What does your financial position have to do with it?
I wouldn't recommend "sending a message" in any way OTHER than by your ACTIONS. Either do it the next time it happens, as I wrote above and without warning, or, go see her in person, look her in the eye and calmly say "If you ever leave our stepson home alone again at 4 in the morning, I will call protective services on you and have you investigated for abandonment."
