Hi Caroline,
Luke is right asking if the kitten has been wormed. I know you say she has but I'd worm her again. We use Panakur paste for 3 days and then again in 3 weeks for three days again. When she is 6 months old you can give her 1 Milbemax or Drontal tablet and she won't need doing again for 6 months.
Don't give her too much chicken because she'll end up not wanting to eat her biscuits
If she has really bad runs, I'd take her biscuits away tonight to give her stomach a rest. Just leave her with fresh water. Then only give her small meals 3 or 4 times a day. Preferably just the biscuits. Just a thought did the breeder tell you NOT to give them milk as this is not good for them. They can't digest cows milk.
Ask your vet for some probiotic paste. Diarsanyl is excellent. I found it clears them up in 24 hours
or you can buy it on line here:
http://www.vetuk.co.uk/index.php?main_p ... ucts_id=30
Hope they get better soon.
Just remembered also Meemonkey mentioned the cat litter...make sure also that they are using similar litter trays, a lot of cats don't like hooded or ones with flaps on. They might prefer a basic square tray.
If they are in a big room put litter trays in all 4 ccorners until he gets the hang of it. They don't like to poo in a tray if it needs cleaning or if there is poo left in from the other kitten