"I believe the software should have prevented the flow of expenses for the 15 year old child (absent marking him disabled), and the tax calculation would then be correct."

Professional tax software is a tool.  Professional tax preparers are still responsible for reviewing the completed return prior to providing the client with a copy.  The software either dropped the ball or you either have an error in the birthdate or a box got checked accidently. Since you loaded in child care expenses for a 15 year old, I am hoping that you entered the wrong birthdate for the kid and thought he was a qualifying child.