The short answer is that you should ask your child for it.
Ideally, you would have have helped your child set up their new Apple iPhone, iPad or iPod Touch when they first received it, and at that point in time, you would have noted your child's iCloud password (also known as an Apple ID password).
However, assuming you did not create the iCloud account at that time, or the password was changed since that date, you will need to get it now.
The best way to do this is by explaining to your child the importance of backing up their Apple device using iCloud. That way, if the iPhone, iPad or iPod Touch is ever lost, damaged or stolen, they can still retrieve all the important contacts, photos, applications, and songs stored on the device. iCloud makes it super easy to restore all of the child's device data through iCloud, and it is very helpful for both you and your child.
The bottom-line is that you need to know your child's iCloud username and password to use uKnowKids for Apple. Once you have that information, you'll be able to quickly set up uKnowKids for Apple for your child's iPhone, iPad, and iPod Touch devices.