First, if you have a baby scale or a food scale, weigh the kittens. They usually weigh about a pound at 8 weeks of age, so that can give you a ball park guess on the age. Second, they need to eat. If they can't eat solids yet, get kitten milk replacer. You can buy it at Walmart, Petsmart, Petco, and most vets will sell it to you over the counter, without examining the kittens. DO NOT feed them cows milk - this will make most kittens VERY sick and is nutritionally lacking. If they are to weak to nurse from a bottle, get a dosing syringe (walmart, pharmacy ect) made for giving little kids medicine and syringe feed them very slowly (going to fast will make them aspirate the milk.
Another reason they may be crying is that they may need to urinate or defecate. Young kittens that are still nursing need to be stimulated to urinate and defecate. This can be done by taking a warm, damp washrag and rubbing their urogenital area (the mother cat would lick this area, so you are mimicking that with the damp rag)
Kittens also need to be kept warm. A heating pad works great, but make sure that there is a towl between the pad and the kittens or they can get burnt.
Good luck! If you have any other questions, please email me and I'd be glad to help you out. If you live anywhere near Silverdale/Bremerton WA, let me know and I can help you out . . . 4 legged neonatal care is my thing!