Whiskas / Felix isn't really a good food to feed them as its full of rubbish.
I found with mine that when they were kittens they liked to try different food, and now they stick with Applaws and Hi-life perfection, as well as raw mince, chicken, lamb and pork.
I tried wet food from Zooplus but they didn't like it - I think they prefer the Applaws & Hi-Life because it actually looks like real meat, and not brains like most cat food!
As for biscuits, I would try and get them on a better one such as:
James Wellbeloved
Burns
Applaws Dry
Porta holistic (Zooplus)
Mine are on Royal Canin which I find really agrees with them but some people don't really like it on here - i guess its what works for your pet.