Awww! Such pretty babies.
DD had a similar experience. Friends heard a kitten crying for a couple days, couldn't find it. Mother had apparently moved the litter and left one behind. DD adopted it and it was so tiny! Had to be fed about every 3 hrs. with a bottle and goat's milk (per vet's recommendation). It is now at least twice as big, just full of P & V, and eating dry kitten food. Is trained to a disposable pad, moving to a litter box next. Now that it is older, we can see it is a flame point Siamese. Quite unusual.
Your little runt looks about the same size. Good luck with nuturing them. Hope they do well. Penny in K'zoo