There are a few things you can try to help your child overcome their fear of flushing the toilet. One approach is to gradually expose your child to the sound and sensation of flushing the toilet. Start by letting them hear the sound of the flush from a distance, then gradually bring them closer to the toilet until they are comfortable flushing it themselves.
Another approach is to use positive reinforcement, such as praising your child when they successfully flush the toilet or offering a small reward for doing so. You can also try to address any underlying anxieties or concerns your child may have about flushing the toilet, such as fears of the toilet overflowing or something coming out of the drain.
It is also important to be patient and understanding with your child, and to avoid scolding or punishing them for their fear. It may be helpful to consult a child therapist or counselor who can work with your child to address their specific fears and anxieties.