Wow.. 28 is not even that bad. I wouldn't have guessed that could be an issue at all. And 10-12 weeks really should be mature as far as I can tell - when holding a 10 week button I don't feel it's skinny, thin feathered or different from an adult in any way.
I'm sure I've had adults outside in...
I'm so sorry you lost your birds! I'd like to learn something from your experience though, since I've had buttons outside during winter with no trouble - how old were the birds and how hard did it freeze? Perhaps you can take a picture of their enclosure or describe it as well, since the...