Day 4 the kittens  have begun to develop their hearing and the remains of the umbilical cord have fallen off.
The kittens are now purring when content, hissing when they feel threatened, they also squeak when they are hungry.

Day 18 some of the kittens have started to test their wobbly legs. They have begun to crawl and to stand.  Some of them have their eyes open. The world will still be blurry to them, as it takes a few days for the vision to completely adjust.  Like many babies, kittens are born with blue or bluish-gray eyes, and as they get older, their eye color will fully develop.