With my little one (we got her at three weeks) we had the same problem.

My vet tech (who has raised countless mommyless kittens) said that we can use the spot (we used advantage) remedies, provided that you only use a couple of drops, and put it on top of their heads, rather than their shoulderblades. Put the rest in a ziplock bag for safekeeping. I refrigerated mine after that, but that was more reflex than anything.

It got rid of the fleas, no ill effects, and I was able to re-dose when the fleas started coming back. Kitten is now a healthy (if ornery and contrary) adult.