Have you asked a vet about it? I don't know about kittens, but I know when I first brought my puppy to the vet at 9 weeks the vet sold me flea preventative for him.
I would just go through a vet. The preventatives they sell might be more expensive but from what I've read and my experiences they work much better and are safer than other options. It would be cheaper/less stressful for you in the long run to treat fleas that way than too buy something that doesn't work and risk spreading fleas to your other animals, or worse make your kitten sick.
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