Did your cat have diarrhea with friskies or whiskas? I don't have experience with Eureka. What is the ingredients? If your kitten have good bowel movement with whiskas or friskies, go back feeding with it & slowly change to new food. Example; 1st day 90% whiskas/friskies + 10% & slowly reduced the old food and increase the new food ratio over the next 7 days.

If your kitten also have diarrhea with friskies & whiskas, you might want hold food for 24 hours. 5 months old is old enough for this. Make sure your kitten always have access to clean drinking water. Then give your cat boiled chicken. You may need to feed your kitten with boiled chicken a couple of times until diarrhea subside. Then slowly feed your cat Eureka.

Coincidently I did read an article this afternoon about Probiotics. It can be given to cats with diarrhea to introduce helpful bacteria into the digestive tract. I can see why you want to try Eureka.
This article may helps: http://messybeast.com/handrear.htm

Mother cat's milk always the best. When the mother cat breast feeding her other kittens, you can try slowly put the kitten near the mother cat so that it can breast feed too. Usually when breast feeding, the mother cat won't mind. Don't leave the kitten unattended because you may need to take the kitten away when the mother cat get angry.