It depends on what capacity you want to work with kids in. Psychologist are mainly responsible for diagnosing and testing, when it comes to kids. They do psychotherapy as well, but oftentimes psychologists aren't covered by insurances and so it's more difficult for parents to pay. Psychologists do make bank, though, once established. Psychologist aren't typically able to prescribe medicine, while psychiatrists are (that's the main difference). Psychologist focus more on studying children while psychiatrists are more focused on the therapy piece.
That being said, you can technically work with kids with as little as a Master's Degree. If I wanted to do child therapy, I could with my Masters in Social Work, so long as I got my license in the state I live in. If I had a Master's degree in Marriage and Family Therapy I could also obtain a license to practice therapy with kids. Same with a Master's degree in Counseling. You can get licensed to practice with pretty much any master's-level degree in the psychology field.
If you wanted to go the full-blown psychologist route anyway, you'll need your undergraduate degree plus a masters/doctorate in psychology (total 10-16 years of schooling depending on how long it takes you to complete each degree). If you want to do research-based work, a PhD is the way to go, while if you're more interested in clinical psychology (being a therapist), a PsyD is usually the route people take. Technically, you can work with kids with both.
All in all, what dictates all of the above is what your goals are and if you're willing to commit to a longer period of schooling. I can always elaborate more based on what your more specific goals are.
As a psychotherapist (whether you choose to be a master's-level clinician or a psychologist/psychiatrist), you have the option of working at a clinic (such as a hospital, mental health clinic, etc.), joining up with other psychotherapists and starting a private practice (which usually comes with time, once you build a client base) or starting your own private practice independently. More often than not, people will work for a clinic until they build up a client base and then work towards branching off to a smaller group of therapists or opening their own individual practice. There are pros and cons to both, and again, I can elaborate more based on what you're concerns/questions are!