Whatever food you buy, buy the one for kittens - it has a higher protein content and will bulk them up.
If your wife can use boiled chicken in any recipes - or starts with a whole chicken for anything - mashed chicken livers or kidneys (boiled) - even boiled chicken shredded - or forget either of those, and just use the boiled chicken broth - mixed with dry food and a little brewer's yeast is another protein booster. You can also mix an egg yolk into the wet food you feed them. You can add a little brewer's yeast to that too. It's all about combining the proteins - the higher quality the protein, the better.
But even cheap supermarket kitten food as opposed to adult cat food will help. But they will LOVE you for boiled chicken broth (we always provided that to them warm on cooooooold winter nights - boiled lots of broth and stuck it in the freezer in plastic baggies, and unfroze it as needed). They will love you even more for boiled chicken livers. And they are NOT going to complain about an egg yolk added to their wet meals.
One thing that's almost more important in winter is water. It can be tough for them to stay hydrated - so that wet meal of food is important.
Found a picture a month or so ago we didn't know we had of Tuxedo. He now lives inside with us - but he was one mean little kitten. Hard to tell in this pic - but look how ROUND and fluffy he is for winter! This was taken in November - around Thanksgiving time - so he was somewhere between 7 and 8 months here:
Feed your kitties with the suggestions, and they'll look like that in no time.