Your son's pixel art is awesome and he should keep it up! Hopefully you guys will have enough time to avoid the AI art next time. Thanks for being honest and forward about it though! I think you have a good base for a full game here. The only big change I can think of would be maybe having a better way to identify the culprit w/o having to rely on the hints? Or some mechanic where you can only choose 1 hint and you have to make the right decision based on the suspects and then choose the culprit based on choosing the right hint. I hope that makes sense!
Hope you and your son had a blast and tell him to keep up that pixel art :D