This information is provided by the American Academy of Pediatrics.  For the complete first aid guide, click here.

Keep your child in a sitting position with his head tilted slightly forward. Apply firm, steady pressure to both nostrils by squeezing them between your thumb and index finger for 10 minutes. If bleeding continues, or is very heavy, call your pediatrician or seek emergency care.

Important note: The information contained here should not be used as a substitute for the medical care and advice of your pediatrician. There may be variations in treatment that your pediatrician may recommend based on individual facts and circumstances.

Source First Aid (Copyright © 2011 American Academy of Pediatrics)