It's always best if momacat feeds the kittens. She may be walking away because their teeth are now aggravating her. But encourage her to feed them by making a big fuss over her and praising her when she is nursing. Give her some treats, like pieces of cheese. Most kittens start getting interested in food around 5 weeks, so try and keep her at nursing for a while longer. If she is absolutely refusing to nurse at all then, you will have to supplement them (KMR=kitten milk replacement) and feed them every 3 hrs. If you're both working and can't take them with you to work, then find someone who can do this for you for a while until they're eating on their own, likely at 5-6 wks old. If you have a postal scale, weigh each kitten. They should be gaining weight every day.