I have an almost 19 year old girl who has always been tiny, but in the last couple years dropped to the under 6 pounds mark. She held steady at 5 1/2 pounds, but I was worried about her, as you understandably are about your little girl.

I found a few things help with keeping her fed, and getting the nutrient levels she needs. There is a high calorie supplement gel that I got at PetCo that I give to her in her food bowl as a treat. It's tuna flavored, so she just licks it up. I also started buying the grain-free foods so that her food is mostly proteins. She does still eat a few foods with grains in them, but it's a mix. These foods are all moist, as well, in cans, and I pretty much feed her on demand (which has evolved to a schedule of 3 times a day - once in the morning, once when I get home, and once before bed.) I also give her treats whenever she wants.

If keeping the other kitties out of the food is important, you could shut her away, or feed her the extra food under supervision.

She may also be losing weight due to the "light" diet of the other kitties.

Definitely check with the vet, but these are the things that worked for us, and kitty actually gained a 1/2 pound recently, which is huge on a teeny animal!