We gave Bruce tripe stuffed cow hooves, he loved them. They are stinky though! So they should be fed outside for sure!
As for putting some weight on, depending on how you mean by "a little skinny" you may not need to worry about getting weight put on. For example, our Bruce was pretty skinny as a pup, and still could be considered "thin" however, you can't see any of his ribs unless he's stretched out laying on his side, and you can feel them if you put a little pressure on, and that is a good weight for growing GSD puppies, it will help prevent future hip problems if they are kept on the leaner side now.
They will also just fluctuate weight, especially that young, and could be about to hit a growth spurt.
However, if they truly are thin, take them to the vet had have a fecal sample tested, and also have her tested for worms, and that could provide you with some answers. But if your vet says that she is healthy and at a healthy weight, then go ahead and stay with what your vet is telling you!
Bruce - Born Nov. 9th 2015, Pick up Jan. 6th 2016
At the Bridge:
Max - With us August 24th 2015 - December 16th 2015