TRACON Controller instructor reported they were distracted when developmental issue a descent into a higher MVA resulting in a CFTT event.

Narrative

Training on Position X in a South Flow at ZZZ. There was very little traffic at this time. The PC12 and one other aircraft were in the airspace. My trainee gave the PC12 Direct to ZZZZZ for the Runway XX ILS from ZZZZZ1 (the arrival fix). This routed transits a 110 MVA. I observed the PC12 at 107 and pointed at the scope. The trainee immediately climbed the aircraft just as when I started to key in. We both assumed this was a pilot deviation. After checking the tapes; my trainee assigned 100 and the PC12 read back 100. I missed this entirely. At the time the aircraft was checking in; I was having a conversation with another controller about a FLC aircraft that was on the TSD (Traffic Situation Display). We had just worked them and I was trying to find out if they were coming back so we had a plan for the upcoming arrivals. Recommendation: All of the things we are briefed on and experienced with happed to result in this scenario. Expectation Bias: 100 is not a usable altitude in this flow. This is not something that is ever assigned. The trainee has never assigned it. Over confidence in the Trainee: My trainee has never had an issue with this. He is new to this sector; however; I have been working with him for months. Very heads up and has never had a box A1 [separation error] after completing 5 sectors.Compliancy: Very light traffic. Guard was let down and I stopped actively listening.My biggest concern is I should have been aware that a mistake like this could happen. We had previously worked some pretty new and complex scenarios up until this point today. Special Military airspace; FLC aircraft; new kinds of clearances; and extra ordinary compression on final. I should have been more aware of my trainees mental state. This is near the end of the day. I should have anticiped him being burt out.