A reputable breeder would have supplied copies of all inoculations and treatments given to the kittens. Or at least would if you request them.
Therefore, Im suspicious of the kitten having worms too.
But stress and new food and environment may be contributing to their diarrhea.
I would save yourself tons of money and forget iams ever existed. Friskies cat food is safer and healthier than iams. Fiskies is just an example, if you are willing to shell out $27 for 12 lb bag of Iams, you should try Royal Canin or similar fine cat food.
Iams is heavy on corn meals as well as “chicken by product meals.” Cats cannot process grains. And is where most of that ripe offensive smell comes from.
Additionally “by-product meals” are basically every part of the chicken, cow or pig that is not meat remaining (beaks, fat, skin, tendons, feet, bone-bits, etc) ground up into a paste.
While traditionally a standard in pet food, “by-product” meals are to be avoided if possible. Especially it is one of the first ingredients on the bag of food.
It may be the same food, but i would imagine the kittens had the same problem before you got them.