I say this every time I start working with a new child or teenage patient: In order for me to help your child the most, I will also be working together with you. Most parents know their children so well, yet they may doubt in their ability to figure out how to solve the child's problem or how to "fix it."
Be it depression, being bullied at school, worrying about exams, trouble making friends or navigating peer pressure, or even more existential worries about life and death, I really consider parents/caregivers to be a tremendous resource when I work with the child.
A collaborative effort is important in so many ways, and I respect the contributions of caregivers greatly. Asking questions, providing examples of behaviors that are of concern from the past or present (even if it is something that seemed inconsequential at the time, such as 'she fell off a chair and bumped her head badly at age 2'), sharing past experiences with other professionals (whether positive or negative)...all these things can and will enhance my ability to understand your child. And of course, the better I understand, the better I can provide strategies for coping and to help to improve symptoms.