I really don't see an issue with it. I would just hit your student hard with atc communications and procedures within class B. Maybe even simulate it/chair fly it with them and pound them hard on ATC communications. Also don't forget to go over light gun signals and lost comms procedures. If you ask your student "what do you squawk if you lose your radios"? If the answer is anything other than 7600 I would not sign them off. You got the right idea going in the middle of the night though. The less traffic and ATC chatter the better for a newbie.