A child that young needs an adult as direct supervision. There is no two ways about it and you as the pilot do not qualify. What are you going to do if the child undoes the seat belt and starts playing with the door handle. 2 year olds are much more capable than 18 months at causing mayhem. If the child can’t keep a headset on they are not ready to fly. If they can they still need another adult as supervision. As PIC frankly this should not need to be explained.
Not the answer it sounds like you want to hear no matter how much you want that to happen.