What was the problem with Fudge at the dentist? How did Peter have to help?

Fudge is Peter’s two and a half-year-old brother. In chapter one of the book, Peter calls Fudge “his biggest problem”, because he is “always in his way”, is messy, and has tantrums when annoyed.
Fudge has to see the family dentist, Dr. Brown, frequently because of the big accident he once had. He fell off the jungle gym while trying to fly like a bird. The fall had cost him his two front teeth. He also had many bruises on various parts of his body. On this occasion, Mr. Brown wants to check out Fudge’s teeth to ensure that they have healed all right. Peter accompanies his mother and brother to the dentist’s office. Fudge is taken to the examination room by the nurse because Dr. Brown does not allow mothers to accompany their children into the room. Later on, the doctor requests Peter’s presence in the room. Fudge has refused to open his mouth so that Dr. Brown can perform the checkup. Dr. Brown uses Peter to motivate Fudge to open his mouth. He asks Peter to open his mouth for a make-believe checkup while praising him for being cooperative and for his beautiful teeth. Afterwards, Fudge emulates his brother Peter and opens his mouth too.