No, unfortunately you cannot assign multiple ticket types to the same seat.
So, as an example, you could not assign a "Child" ticket type, and an "Adult" ticket type to the same seat in a seating chart.
How can you offer such a solution? The answer, is a discount code. Since only 1 person can sit in 1 seat at a time, if a user purchased an "Adult" seat, you could offer a children's discount code to essentially give them the same rate as that Child ticket type.
However, be careful about doing a method like this, because you would not want to have people abuse this power, since you have no way to validate if the seat will actually be used for a child or not. This is just an example, there are other reasons this solution might work for you.