I downloaded the largest size available from flickr.
There are two completely separate issues here.
The faces can be "brought back" with the DODGE tool. I suggest starting on the midtones at 25% strength and building up to the skin tone you want. Small, feathered brush. Leave the hair alone.
The sunlight is another matter. Generally, it's possible to compensate for under-exposure but once an image is blown it's blown. You can't bring back detail that was never recorded in the first place.
The exposure actually isn't TOO far off. There's plenty of detail in the white t-shirt. It's the position of the subjects that's the main problem - i.e. shooting straight into the Sun.
The image posted on flickr is JPG. If there's a RAW version you'd have a better chance but it'd still be a very slim one.