Parents are a child's best - and often only - advocate.
There are times when a child stands accused of something and their voice isn't heard. Accusations tend to carry more weight than facts. A little person who stands accused of something is less likely to get a fair hearing, especially in the face of heinous or hysterical charges. A child on the defensive needs an advocate to level the playing field.
There are also times when a child's situation is misdiagnosed. Children are subjected to all sorts of mechanical social processes, most prominently the education system. The machinery of "the system" doesn't handle exceptions all that well. For example, a child that appears to be struggling with a subject may actually have mastered it. Lackluster performance may be a result of the fact that they're bored to tears, and subsequently not engaged. Unfortunately, those administrating "the system" know only to assess performance in a few set patterns: the kids studied this subject, then took this exam, so that exam score is the true barometer of their subject mastery. These mechanical processes aren't designed to recognize, let alone deal, with exception cases.
When an authority presents you with a conclusion about your child, you can't take it at face value. You have to interrogate everybody involved to get all the relevant facts, consider alternative conclusions that the data might be trying to tell you, find ways to test for the truth, and engage everybody in ferreting out the truth. It can be laborious. Advocating for a single person runs counter to an educational-industrial complex designed to mass-educate large volumes of children. Also, enabling those who drive a mechanical process every day to be meta-aware is no small task. No surprise, flying directly into the headwinds is emotionally trying. But the parent-advocate must be prepared to do this and keep a level head in the process.
There is a dark side of advocacy, of course: the parent who, even in the face of overwhelming evidence to the contrary, asserts their child can do no wrong, is perfect in every subject, is the greatest athlete, is the most talented musician, etc. Delusional advocacy is quite damaging. It creates a false floor of performance, a sense of entitlement, and a wildly distorted sense of self-awareness. That doesn't build self-esteem; it creates a recursive pattern of denial.
But that aside, a child has no better and no more important an advocate than a parent. An attorney is a hired gun, there to argue points of law interspersed with selected facts to win an argument. A parent is invested not in winning the argument of the moment on behalf of a child, but in championing a child's cause in life-impacting events.