Give it another day, and then check again,.the mother might be around looking for food for herself, and coming back to feed the kittens. Cows milk isn't suitable for baby kittens. Are they in a covered space, if not, perhaps you could find a plastic box, lay it on its side with the open end facing towards you, so that it looks like a kennel. Put an old towel inside and put the kittens on it. At least this would protect them from the weather and the mother would be able to get inside to feed the kittens. If you are still not happy after that, and you feel sure the mother is not coming back. Then if you took them indoors, you would need to get kitten formula and a feeding syringe from a vet. They would need feeding at least every 3 hours, and if you managed to rear them successfully, then you also have the problem of finding homes for them. Good luck.