When is it necessary to feed a baby kitten with a baby bottle?

You don't bottle feed your kitten at all, if your kitten has a mother, it will naturally drink its mother's milk.

Better answer:

If you absolutely needed to bottle feed a kitten you could give it "Hartz precision nutrition milk replacer for kittens", or any other brand of kitten milk replacer. Try to let the baby nurse on mom for at least two days though before trying to bottle feed, and never forget a baby kitten cant use the bathroom on its own, so you need to use a warm wet rag and gently rub genitals to help it eliminate. At 5 weeks you shouldn't have to help it go potty anymore. Many people try bottle feeding baby kittens or puppies and then wonder why the baby dies, and it is normally because the owner does not know you have to help them go potty. I really hope this helped! :)