If the infant or child is not breathing and you have help at hand then send the help to contact emergency services. If you are on your own then administer 5 initial rescue breaths followed by 1 minute of CPR, prior to contacting the emergency services yourself.
If you are on your own then take the infant or child (if possible) with you whilst you contact emergency services. In the case of a child try and locate and AED, Preferably with paediatric pads.