While the app itself is free, you do need to have an Oral B or Crest product in the bathroom to be able to activate it.
The free Disney Time App works by scanning a Crest or Oral B product which then activates the app to uncover hidden Disney or Marvel characters, which they can then save in the album within the app, collecting all their favourite characters.
The idea is that kids will brush for as long as it takes to uncover the image, but I'd suppose adult supervision will be required. I know my 5 year old will easily find ways to mess with the system!
It looks like they had a few problems with the app back in July last year, but these should be fixed now and I tried the app with both my girls tonight, and they loved it. Hopefully it will encourage longer brushing!