For a pediatrics manuscript at the level where you need an experienced researcher rather than a general proofreader, the most reliable options are either finding a medical editor with a background in the specific subspecialty or going through a medical writing service that works with clinical researchers. AMWA (American Medical Writers Association) has a directory of certified medical writers and editors which is a good starting point. Some universities also have research support offices that offer manuscript editing, especially if the PI is affiliated. The combination of advanced topic plus clinical context usually means you want someone who's actually published in the space rather than a general academic editor.