You need to weigh them and the mother. Today, 3 days from now, and if they have gained enough weight (or stayed the same for the mother) you can go to ~5 days between weighing. They should start over 25grams, preferably 35, and gain 2-4 grams a day. If not you need to supplement them with handfeeding or if it's just one give it some time alone with mom. You also want to keep an eye on mom's belly. If the kits are overzealous in getting milk they can cause irritated nipples that make her not want to feed them and sometimes chew her own fur off in that area. Watch the bar spacing because anything a kit can stick it's head through the rest of it can follow and remove any high shelves so they can't get on them and fall off.
