Paxton's Pad is a perfect transition bed that can last for years.  Simple, clean and safe for kids.  Rails can be all around, just one side, or none at all.  Slats can be added for extra support ($). 
 
Size pictured: Twin
Prices START at $500, go up with added slats/rails, etc
 
I can do other kid floor bed designs so stay tuned or reach out with your concepts :)