Disney's CFO mentioned that the MVNO was having some challenges. (See Silicon Alley article)
I don't think there is a lot of news here. Going in, I'm sure that they knew there would be a lot of challenges and scale + customer acquisition would be a challenge and they would need to commit to the venture for years to see it pay off. One of the other factors not mentioned in the short piece was network effects. There are many emotional reasons that parents give for getting cell phones for their children with cost making it an easy decision in the end – bundled minutes, bundled messaging, etc. One of the challenges is – once kids are old enough to be heavy users (and attractive to carriers), parents tend to put them on family plans. It's younger children that more typically fall into the category of being on prepaid plan.
Disney Mobile has launched a number of very innovative services that really hit the mark in terms of what parents want.
For these reasons, it would be sad to see them disappear after such a short time period. They seem to be doing a good job of acquiring customers – it just takes time.