Sending text to phone when child leaves the house

Hey guys, my neighbor has a down syndrome kid that ran out of the house and later found him half a mile away with blisters all over his feet because of how hot it was. They were wondering if I could make them something that would text there cell phones when he leaves the house. I was thinking they could attach a rfid tag to his shirt or something and when it goes out of range, it will alert the cell phone. Would this work? If so, what would I need to get to make it as cheap as possible? Thanks.

I decided that I'm raise an alarm if he leaves the house. Then it wouldn't be too expensive.

I would monitor the doors, optionally the windows?