I'm not sure about electives specifically in the third year, but if you follow the link that Blade28 provided above, you can pretty much contact any medical school and apply for an elective in their children's hospital. Do you have a specific children's hospital in mind? I know that most of them offer electives through their partner medical schools.
A word of warning, though - depending upon when you want to complete the rotation, the deadline to apply may be looming ahead. Many of the applications are annoying in their requisites, requiring a letter of support from the dean of your med school, verification that you've completed all required rotations for the elective, immunization records, etc.
Good luck.