First, we need to calm their tummys down. It could be a variety of things causing this, but I would like to try a couple of things before deciding if they need to see the vet. I would withhold all food for 12 hours. Put Pedialyte (unflavored) out for them to drink in place of water. They may be a bit dehydrated from the vomiting, so we want to keep that under control so it doesn't get out of hand.
After they have been off food for 12 hours, do one of two things - either boil a chicken breast with some white rice and run them through the blender with the broth to make a puree for them to eat. This is considered a 'bland' diet and has nothing that would make them ill. You can mix a couple of spoonsful of plain active cultured yogurt into this also, to put natural flora back into their system.
If you don't have a chicken breast, you can get some plain Gerber Baby Meat - chicken or turkey and mix that with the yogurt and give them that. Keep them on the bland diet for 24 hours and see if they are able to hold the food down.
I would throw out the food that you have they got sick on. It may be a bad batch of food that somehow got tainted and that could be the problem. Buy a new box/bag and start them back on just a little of it mixed with the chicken on day 2.
If they are still vomiting, you are going to have to take them into the vet. If they hold their food down, you can gradually add more of the dry food to their babyfood until they are eating all dry again.
Please let me know if I can help further.
Feline Healthcare
16 yrs health care mgmt & issues relating to cats, reproductive issues and multicat environments