To answer your first question, I realized pretty early on that there is just far too much information to be able to know it all. You have to strategize and figure out which pieces of information are more important for you to know and which ones can take the back burner. I felt uncomfortable going into my exams feeling like I hadn't memorized absolutely everything but I'd come out on the other side realizing that medicine is more of an art than a science and that being able to synthesize the importnat points is a more valuable skill than being able to regurgitate every memorized fact. So with that in mind, my strategies have been more focused on how to integrate material learned from different lectures and professors. I study things in groups based on an area of the body for example. Many schools will have separate courses that teach physiology (healthy systems), pathology (diseased systems) and pharmacology (study of drugs) as different entities, but I usually try to take the information I've learned about the heart, for example, in each course and learn about the system, its diseases, and its treatments all at once, since that's how we will encounter it clinically in our careers anyway!
If you're interested in pediatrics, it's never too early to start shadowing! Reach out to pediatricians in your area and ask if they'd be willing to have you along for the day when they see patients - most are more than willing.
My preparedness in medical school came from my solid foundation of sciences coming in. I think this is something I gained even tracing back to my time in high school, where I was constantly pushed to stretch my mind and go one step further. The same was true in college, where I was able to choose my courses within my major (Biology) and was encouraged to take courses that aligned with my interests. This made me more excited about what I was studying and helped me develop a strong foundation in the sciences, which was strengthened further by my time completing my Master's degree last year. If you have the freedom to pursue sciences in the areas that interest you most, it makes learning about other areas in the future that much easier once you've mastered how to study and how to learn!