I haven't had a lot of experience with RFID systems, but what little I do have/remember: In order to get the range you need, you'll need to go with higher frequencies which equates to higher costs. I think 915mhz+ will do 10'? Something like that. It's been a good while since I've looked into it though. I'll check around the webz a bit.
You should just have them all have iPhones or Droids and make an app for that! 
