I'm asking for a friend tonight. Her sister just got an 8-week old kitty. (not sure how that came about) She said the kitten is in need of a bath but we're not sure what is safe to bathe one that small with?
Use a little Dawn dish soap and wash in the kitchen sink. Be sure to rinse well and have a nice warmed towel ready to dry off. Just be sure the kitten is kept out of any drafts, A/C, etc. till dry.
I've washed 5-6 week old kittens with no problems.
You could also use a shower, I prefer showering cats as it seems to be a lot less stressful on them.
Bathing is ok too, as GoldenKitty said use the kitchen sink or a handbasin.
I too have bathed a kitten as young as 5 weeks old no problems. Infact starting them off at a young age makes it a whole lot easier down the track.