It's hard to imagine anything more enticing than a fuzzy, warm, newborn kitten. If your cat's agreeable, you can handle her newborns days after their arrival. Your attention will be their first human interaction and will set the stage for their socialization, so proceed gently and cautiously.
Newborns In Development
Lady's kittens are totally dependent on her the first two or three weeks of life. Many of their senses are still developing; their eyes are still closed, their ears are still folded and they aren't all that steady on their feet. They haven't developed bowel and bladder control, so Lady has bathroom duties to perform, in addition to feeding and grooming chores. During the first week or two, kittens respond to their mother through their sense of smell and touch, as well as by feeling her body heat.
Benefits of Human Interaction
As kittens grow and their senses mature, they also begin to learn behavioral and emotional lessons. Human interaction at an early age tends to increase sociability toward people. The ASPCA notes careful rearing early in their development helps kittens mature into well-adjusted cats. The organization also states kittens who are handled more during their first two months are more likely to be friendly with humans as adults and physically develop faster. FAB Cats refers to the period between 2 and 7 weeks of age as the "sensitive period," when particular events are more likely to have long-term effects on kitten development.
Your relationship with Lady will guide your interaction with her newborns. Some new mothers are very protective during the first few days after giving birth; others welcome attention from their human friends. During the first few days, if Lady doesn't mind, try to stroke the kittens gently with one finger on the top of their heads and backs. Pick up the kittens one at a time, give each one a gentle stroke and turn her over for a couple of seconds and then return her to her litter mates. Keep your interaction brief -- about five or 10 seconds should do it. If Lady becomes anxious over your attention, wait until she leaves the room for a snack break. If you sense she's not pleased with this attention, back off and wait until the kittens are about 2 weeks old before you begin to handle them.
Two Weeks and Beyond
As the kittens grow older, you can interact with them for longer periods of time. By the time they're 3 or 4 weeks old, their eyesight and hearing are fully developed and they're walking better, so they'll be more interested in engaging with you. Keep picking them up, petting and cuddling with them, so they can get used to human touch. Allowing them to experience a different room -- perhaps carpet instead of tile -- and exploring different objects, such as boxes, paper bags and play toys, will expose them to different sights, scents and sounds, building their confidence. If you don't have time to play with your kittens, gentle petting for a couple of minutes a few times a day will go a long way to help develop their social skills.