A GPS Tracker will do the trick- when set up, you can see where the person is on your mobile phone etc. It has to be recharged from time to time, and you do need to be a bit techy to set up the sim card-must be 2g compatible network), but there are no other charges with this one other than the Sim card data charges. Do a bit of Googling to get familiar with what you need to know to set it up(or ask any young person you know to help!) You can also return it to Amazon if you don't get on with it. Here is a link , have a read of the reviews- good luck
Amazon.co.uk User Recommendation