Firstly I would be having a few strong words with the person you got them from about irresponsible breeders and getting her cat spayed immediately
If they are under 8 weeks then they may not have been introduced to solids properly so you will probably need to mush the food down for them, and dilute it to make it easier to chew. After they have eaten, then place them on the litter tray (use a finegrain litter, not a coarse one) and stroke along their backs and tummies. Female cats do this to stimulate kittens to pee and pass motions when they are little. Keep them confined in a small, controllable space. It will a) help them feel secure and b) limit their opportunities to use places other than the litter tray for doing their business.
Be patient. They are babies and need to be given the same care, and loving support that you would give to any baby.
If they have not eaten in 24 hours, see a vet, as small kittens can fade very quickly.
Make sure they have access to water at all times.