Find out what they have had so far - a good breeder will have already started a worming program. Take them to the vet in the first few days for a check-up, many vets will offer a pre-vaccination check for free, and to get them weighed. They need to be a minimum weight of 1kg for most of the spot on flea treatments including the advantage - and check it is advantage 40 for small cats and kittens, not 80. My preference for routine treatment of kittens is advocate spot on, which is both a flea and roundworm treatment. If they actually have fleas then they will need a tapewormer so a combination of advantage (which is just a flea treatment) and drontal or milbemax would be better.
Firstly Advantage is safe on kittens from 8 weeks of age. Drontal is safe from 6 weeks of age though you should weigh the kittens if individual kittens weight less than 2Kg (they probably will) they should only have half a tablet. I would go with a list of questions for the breeder of the kittens about have they been wormed or flea treated. Ideally we start worming kittens from 5 weeks of age so the breeder may have already started doing this. Personnally I prefer milbemax for kittens as they do a specific Kitten tablet which allows more accurate dosing 0.5 to 1 kg half a tablet and 1 to 2kg a whole tablet. Plus the tablet is a lot smaller than drontal.
We are getting kittens next weekend (9 weeks old) and I was wondering when I need to flea treat/worm them? They won't be going outside yet. Should I worm them (Drontal?) straight away (and how often?) and what about flea treatment-I used to use Advantage on my cat, is that ok for kittens? Should I be treating fleas as a preventative measure or should I only treat the if they actually have fleas when they are that small?