My daughter took her four chidlren to see a dentist. The 8-year-old had no cavities. One of the 6-year-old twins had not cavities. The other 6-year-old had one cavities between a molar and cuspid. My daughter was devastated that her child had any cavities. All her children brush at least twice daily and use floss as well. She used a wash cloth to clean her baby's mouth before she had teeth. The baby is now 1 and has six teeth, no cavities.

The dentist told my daughter that the tooth would have to be capped. My concern is why? My daughter cannot see the cavity and it does not bother her son.

Why can't the dentist just drill and fill the cavity?

The only reason a baby tooth would be capped would be because the decay was so invasive that it would be the only way to keep the tooth in place until the adult tooth was ready to come in. If mom can't see the decay it can't be big enough to warrant a cap.
Perhaps a second opinion is needed?