The best worming tablet to use is Drontal (only available from vets). You should only need half a tablet per treatment. Those available in pet shops, supermarkets etc. aren't very effective and don't kill all the worms. They also tend to involve the cats taking several tablets over a period of weeks, depending on which you use. If you really can't get the tablets down your cats the vet should be able to do it, showing you a good technique.
Drontal is very bitter and wouldn't be acceptable in food - it makes the cats drool a lot as soon as they taste it. It is a very good and thorough wormer, however.