I don't have the experience as a pediatric or emergency nurse but nursing itself is very demanding and each speciality has its own challenges. Why don't you volunteer working in a pediatric clinic or emergency department to experience what they do. In turn this will help you decide what would be a best fit in regards to working as a pediatric nurse or an ER nurse. Good luck.
These are both very competitive nursing jobs but it also depends on your state and close you are to the city. The best way to increase your chances are to work as a nursing assistant at the hospital you want to get hired at or make sure to make a great impression during your pediatric clinical. Best of luck!