What to do when a kitten throws up phlegm?

How old is the kitten?

What are you feeding it?

The kitten should never have cows milk. If you are mixing milk with kitten kibble it should be kitten formula from the pet supply or goats milk from the grocery store. Many cats are lactose intolerant so never cows milk.

Kittens should not eat adult cat food until they are an adult cat at one year old. Canned kitten food or kitten kibble only. That food has extra vitamins minerals and calories that growing kittens require and is not in adult cat food no matter what the label says.

Has the kitten been dewormed they all must be.

Ask you vet about vomiting. If this happens after every meal it is not getting the nutrition it needs and there is a problem. Its not normal.