How to make a GPS tracker?

I've just taken my three year old daughter on a day trip to London, we briefly got separated in the crowd, can you recommend a small short range GPS tracker / locator to make finding her again easier?

• While I take the precaution of putting cards with my name and number in her coat pocket and bag, these rely upon somebody finding and helping her. Being the paranoid parent, is there a small simple smart phone based locator I could use if my three year old and I get separated in a crowd. While we are out and about she normally wears a rucksack with a wrist strap for me to hold onto, but this is quite restrictive for her, so I'm looking for options as she gets a bit older.

• Answer:

  First, you have my sympathy for the feelings you must have felt during this brief moment. Second, I want to be provocative: DO NOT over-react. I understand you found your daughter quickly, and she was safe. No harm done. Make the experience a learning opportunity.  For her, she should learn to remain calm and call out for your name if she gets lost again. For you, learn that you cannot control everything, that your daughter should be as autonomous as possible so she can take care of herself.  In parenting as in everything else, problems are inevitable. Don't forget that problems are solvable too. (I strongly second the idea of putting cards with your name and number in her coat, btw.  Teach her to use those cards with confidence.)