Do get a second opinion, but yes, kids can get cavities. My almost 5 year old nephew had 4 cavities taken care of last summer when he was 3. The pediatric dentist was able to take care of 1 in the office while he had to be put to sleep at the local children's hospital for her to do the other 3 in the OR (he's got a history of breathing issues). In the end, 2 had to be pulled, 2 were capped, and while in the OR, she found an additional 2 teeth were affected and taken care of.
Dental issues are serious. If there is an infection, it can get into the bloodstream and cause serious health issues (even fatal!). Also, if the baby teeth are not properly cared for, the adult teeth can have problems when they come in.