I like your use of puzzles for teaching. For a philosophical and learning experience, I agree that an increase in the number of pieces shouldn't make it more difficult. Even though some pieces have the same shape, they have a specific purpose and place. Doing this should also help her develop more patience. Once she finishes it, the feeling of hard work paying off should be very satisfying.

But in a literal and actual sense, the number of pieces does make it more difficult.