Can I track my kids iPhone without them knowing ?

Actually, the Find My Friends App requires approval of your request to add them.

Apple is not going to enable stalkers worldwide by giving them the ability to track iPhone users without their knowledge. That you are a parent is irrelevant.

I believe Apple's strict policies around privacy do not allow the type of undisclosed monitoring you are looking for.

You can through the Find My Friends app -- About Find My Friends - Apple Support.

Log into iCloud.com using their Apple ID and then Find My Phone.