You can use clove oil. It's sold in pharmacies as a remedy for tooth aches. It's an anesthetic, so they simply feel a pleasant tingle and go to sleep.
You need to separate the hatchlings into a smaller cup with a low water level. Pour a bit of the oil into a SEPARATE bottle, add a bit of water to the bottle, then shake thoroughly. Oil isn't water soluble, so mixing it like this before adding it to the hatchlings is
extremely important.
After it's thoroughly mixed, pour a few drops into the container with the hatchlings and gently swirl the cup. This will make them a little drowsy. Then gradually add more of the oil solution as needed, pausing to gently swirl.
It's very fast, and very easy. There isn't any violent twitching, and no suffering. It's not fun or pleasant, but it's the easiest way I've heard of, and having had to do it a few times myself, I can tell you that it's much much kinder than letting them starve to death or anything.
An important note: the containers you use to do this in need to be used only for this; I wouldn't use the same containers later for more every-day purposes. There are probably ways to clean it well enough that it wouldn't be dangerous, but this is my own paranoia. I would devote containers specifically to this purpose.
Good luck
It's not a pleasant part of keeping axolotls, but it's necessary. (Also I would say this is only appropriate for younger larvae. Older juvies and certainly adults need to be taken to a vet for humane euthanasia)
