It's never a good idea to switch foods on young kittens.
They should start being wormed at 5-6 weeks when they begin eating. If you can get hold of a bottle of pyrantel at a pet store, it will give you the dosage. Then worm again in ten days.
Are the runs simply liquid poop, is the poop very light in color, and does it have a bad odor? The combination of those three generally means there is a bacteria at work, and they need antibiotics of some sort.
The most common cause in kittens is either infection, of a mother who is starting to cycle again and they pick up hormones through her milk that make then loose.
I would worm all your cats first of all. Then for your adults, get a can of pure pumkin, NOT pie filling. Give each of them a teaspoon of this morning and night till the runs stop. This is full of fiber that will absorb the excess fluid in their bowels.
You might also get some Kaopectate, especially for the kittens, because diarrhea can lead to dehydration and death in a very short amount of time. At 7 weeks, I could say 1cc or 1ml of Kaopectate each, three times a day. I would be hesitant to give them pumpkin and upset their diet as well.
I start my kittens out on canned food, one bland flavor, until they are 8-9 weeks old when they get into the adults dishes. This keeps their stomachs from becoming upset, or getting the runs from a change of diet.
Good luck and let me know how you make out.
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Sea hags have feelings too. Honest!!!