I hope it's not too late... but here's what I would do:
Go to pretty much any pet store and buy KMR (Kitten Milk Replacer) and a syringe for feeding... and a set of rubber nipples, like the ones you would put on baby bottles (they will usually have these kitten-sized). Mix the KMR as instructed on the can, and warm it in the microwave as you would for a baby... not too hot, not too cold. Fit the nipple on the end of the syringe... or go without, if they'll take it (it just feels more natural to them with the nipple, quite often), fill the syringe with formula, and GET AS MUCH INTO THEM AS YOU CAN, WHETHER THEY RESIST OR NOT! If they won't open their mouths, it sounds mean, but you need to pry them open and get some formula into them. Just give them a chance to breathe, and only give a very small amount at a time, to prevent aspiration. If they don't eat, they won't survive. You should be feeding them about every 2 hours, night and day, and after feeding, stimulate their anal areas with a wet, warm facecloth or rag to stimulate them to urinate and defecate. Their mom would be doing this for them by licking them if she was raising them. It is important to keep them warm, as you said you have been doing, but never put them directly on the heating pad, always keep it on the "low" setting, and give them enough room in their box to crawl away from it, if they get too hot. I've fosterd lots of young kittens for the SPCA in my area, and have never had an orphan die. Do be prepared, however, for mortality... especially if they haven't gotten colostrum from the mother within the first few days of life. You sound like you will be good "kitten parents"... good luck!
For some more in depth information on raising orphans, here's a pretty good article: http://www.peteducation.com/article....&articleid=912