I have a "Find my iPhone" app on my iPad, but since I don't have an iPhone, I've never bothered to look into it.
Unless there's a way to do this through a Web site, this suggests that you can only do this from anothe iThing. You also have to be able to log in using the Apple credentials for the missing device.
There are a couple ways to do this If you also have an iPhone you can set up a feature called "Find my Friends" and you can see where the phone is in real time.
If you do not have an iPhone and your child's phone has Find My iPhone enabled, you can go to iCloud and log in with his or her Apple ID and locate the phone.