You will likely be required to work alongside a dentist, so your interviewer will ask you this question in order to gauge your comfort level. Although it is okay to express a preference for one over the other, you should be able to provide an answer that portrays your ability to work with either age group. "Although I especially love working with children and keeping them excited about good oral hygiene, I am just as comfortable working with adults" is a fantastic example answer. Although you may not be assigned to work primarily with children, your preferences will likely be noted if you are hired.