It's not inevitable that a child will develop a cavity in one of their primary (baby) teeth. That being said, it's not exactly rare. Of course, it's not a comforting development, but it's not a reflection on your parenting. Your children's dentist can help you to refine the best oral health plan for your child, hopefully preventing future recurrences of tooth decay as much as possible. But for the moment, you have to help your child prepare to have that cavity filled. What's the best way to help them get ready?
Previous Visits
How has your child reacted to their previous visits to the dentist? This could be a decent indicator of how their upcoming appointment will proceed. Some children are nervous patients, and others handle the experience calmly and cooperatively. However, a standard checkup is quite different from having a cavity filled.
A Calming Effect
Sedation dentistry is available, and at this level generally involves nitrous oxide (laughing gas). This is more of a relaxant than a strong sedative, and it simply calms a nervous young patient as needed. If you suspect that some form of relaxant is going to be helpful (or necessary), have a talk with your child's dentist prior to their appointment. Minimal preparation is needed for using nitrous oxide during a dental appointment, so it can be decided and arranged quite spontaneously if required. Numbing your child's jaw is another option, but will require an injection.
A Positive Experience
A calmer experience isn't guaranteed but can be made more likely if your child knows what to expect. Frame the experience as positively as possible. You don't want your child to interpret their dental treatment as punishment for something they may or may not have done with regard to their teeth. Having the cavity filled can be depicted as a beneficial task. If your child's cavity has caused sensitivity and discomfort, you can emphasize the fact that their treatment will make their tooth feel better again. As much as you can, make the upcoming appointment something to look forward to.
During the Procedure
As you would know from having cavities filled yourself, the experience isn't distressing or painful. A lack of familiarity with what's involved may be a little unsettling for your child, but you'll be there during the (brief) procedure for some emotional support. Your child's dentist can further explain the procedure before they begin, and knowing roughly what to expect can be very helpful for your child.
A cavity in a primary tooth is far from ideal, but it creates an opportunity for better oral health in the future, since even though the experience was as calm and straightforward as possible, your child probably won't want to repeat the experience. Contact a children's dentist to learn more.