When your child turns 17 and acquired a driver's license, you need to add your child to your auto insurance policy. It is important to do so because every regular driver is required to be added to your insurance policy. Usually, the overall premium is also lower than a separate car insurance policy for your child.
The information in this answer is general in nature and is not legal or insurance advice for your specific situation. Policy language varies carrier by carrier; your declarations page is the final word. If you're unsure, call your agent before you need to file.