It's mostly because of all the pressure. If you think about the most well adjusted celebrities they usually think of whatever field they work in as just a job and keep very private lives, often staying away from social media. Cillian Murphy, for example, doesn't like the fame that comes with being an accomplished actor. He's an actor because he enjoys acting, not because he enjoys the attention.
Child actors are more vulnerable to this sort of external pressure from fans and the media because they don't know how to set up and enforce boundaries. Their lives and their mistakes as they grow up are widely reported on and scrutinized. It's no wonder the pressure so often gets to them and they end up with mental health or substance abuse problems. Daniel Radcliffe managed to avoid any issues because he had people looking out for him and was able to lead a private life while working on low profile and more out there projects he genuinely was interested in. His co-star, Emma Watson, distanced herself from acting and focused on her studies.
Contrast this with celebrities like Britney spears and Amanda Bynes, both of whom were exploited and abused. Their mental health struggles were widely reported on and mocked at the time thanks to the tabloids and it's only now that we know the truth of what brought on their respective issues.
In the end, money is great but it does nothing to protect people from the mental health struggles that we all face and the fame and overbearing attention can make things worse. The entertainment industry can be a vicious and exploitative place, especially for people who are already facing their own inner struggles. If you want to write about this topic, I recommend researching some of the cases of actors who've had a difficult time and see what sorts of things they've had to endure.