Below the protective layers of enamel and dentin, your child's tooth has an inner layer of pulp that houses the tooth's nerves and blood vessels. When a cavity reaches this layer of the tooth, the pulp can become inflamed and very painful.
Pulpitis can be classified as either reversible or irreversible. If your child has reversible pulpitis, the tooth will be painful to the touch. The pain may come and go, and it can usually be alleviated with over-the-counter pain medication. However, with irreversible pulpitis, the pain may be constant and severe, and may not respond even to painkillers. The tooth might also be mobile.
Your dentist will perform an X-ray to determine the extent of the damage to your child's tooth and whether the tooth can be repaired. Unfortunately, extraction of the tooth may be necessary if it's not repairable. Depending on your child's age and where this particular tooth is located in their mouth, temporary replacement or space management decisions may be necessary for their permanent tooth to erupt properly.
The dentist's next decision is to determine if the tooth's nerve is still alive and can be saved. If so, they may perform a pulpotomy after removing the decay. If the nerve is dead, however, it may require a root canal (pulpectomy and restoration) or extraction instead of a pulpotomy. In some cases, your child's baby tooth may be ready to "shed" or "exfoliate" anyway, as an adult tooth is about to erupt. If this is the case, it makes sense to extract it.