Here our friends at Sheffield Children's Hospital have made a great video giving advice on wiping
Best way to sit on the toilet
A child’s posture is important with toileting to support with balance, and it can also support with bowel movements. Some children may need a small step at the front of the toilet to ensure their feet are grounded, or may need a smaller toilet seat to support with stability.
Children’s Therapy (Physiotherapy and Occupational Therapy)