It wouldn't show up in Find My iPhone using your Apple ID unless Share My Location is enabled in your child's iCloud settings.
Check the Restrictions settings, there are several that might interfere with Find My iPhone.
Make sure your child's account is fully signed into iCloud - sometimes it seems to be signed in when it isn't. Try to access the Password & Security settings, see if you're prompted for a password. You can also try a sign-out/sign-in to iCloud.