This theory suggests that we have an inner drive to hold all our attitudes and behavior in harmony and avoid disharmony (or dissonance). This is known as the principle of cognitive consistency.
When there is an inconsistency between attitudes or behaviors (dissonance), in this case littering something must change to eliminate the dissonance.
Mum should reward her daughter, everytime whe refrains from littering, and in this way, she should eliminate the dissonance. An appopiate response would be to raise his daughter's allowance whenever she refrains from littering.