Well, there are many answers to this question. The obvious one to tell your child is to be their parent. Eventually, they wont believe it, and start looking for other jobs.
Doctors are important, they cure the sick (or try to)
Engineers are important, for they design the machines we use.
Authors are important, for they inspire, as well as design whole worlds.
The list can go on. It's really their call for which one is most important.