Cavities can be very painful on their own.. Now dentistry for children is much different than it used to be,. The Dentist will explain everything to your child and your child will be fine.
My niece had to have 4 cavities filled after her first visit!~ We were all so worried and she did great.. But it also taught her right away that she was always going to have to take extra good care of her teeth in the future, because she was more likely to end up with cavities than many of her other friends.
I know I always worried about these events and was always surprised at how well our daughter did. I would just try to stay as calm as possible and not make up stories about what it would feel like, since I did not want to tell her it would not hurt in case there was a little pain. I would tell her I would be there but I would need to sit quietly while the Dentist and her assistant were doing their job.
I also always took a book to read to our daughter while we were in the waiting room. I would start it a few days before so that it was a continuation of something she was familiar with.. IF the Doctor or Dentist allowed, I would read during a procedure or at least in between procedures..
I am sending you strength..