I’m sorry two of the babies died. That’s very common with first time mothers. They don’t know quite how to be mothers.
Its good that you hold the surviving baby to be fed. A bunny typically feeds her kits 1-2 per day, for a couple of minutes each time. Rabbit milk is extremely fat so the kits don’t have to feed often. Between feedings, the kits lie by themselves, wrapped in the mother’s fur. The mother doesn’t lie with them like cats and dogs do. You should give the kit a nest box so it doesn’t crawl away and get cold, because that becomes fatal very quickly. Is the kit wrapped in mama bun’s fur? The mother should pull out fur from under her chin when the babies are due, because the fur comes loose at that point in time.
If the baby’s tummy is nicely rounded after feeding, it’s getting enough food. That the skin is wrinkly is normal, as long as the tummy is rounded.
Tis is just a reminder of our forum rules: Binky Bunny doesn’t allow discussions about intentional breeding. If you are planning to breed bunnies, there are forums that better cater to your needs. Rabbits Online is an active forum that’s frequented by people with a lot more bunny breeding experience than most of us here have. This is a link to Rabbits Online, the relevant forum-section: http://www.rabbitsonline.net/forums/the-rabbitry-and-show-room.20/