When would be a good time to intership since I wanna become Peadriatic Nurse?

Wonderful choice!! it's never too early to start at least volunteering at your local hospitals. With the major insttitutions we have in my area, you can express an interest to volunteer in certain areas and I would imagine the hospitals in your area would offer the same. This will allow you to become better acquainted with the day to day atmosphere and workings of a hospital, and if you do volunteer in the pediatric wing, we can see how it functions daily and even be able to garner some valuable knowledge and working experience from the nurses on staff.
Internships usually follow once you have actually started a study program. Those opportunities are usually explained at orientations. If you have a couple different schools in mind, spend some time reaching out prior to making your final decision on what kinds of work study or internships might be offered between the school and local hospitals. Many hosptials like to work closely with specific schools for their placements.

Yay! Pediatric nurses are great! I personally never did this -- but I do know that many hospitals offer what are called "shadow days" for teenagers/young adults who are looking to experience what life would be like as a nurse. I highly suggest that you research on the website or call your local hospitals and see if they have pediatric units, I would then ask if they offer a shadow day for students. You also might be able to experience other units, as well. I hope this helps!