Question

Why were the first years of a slave child's life relatively good

Answer 1

Because no one expected them to work too hard and they didn't have children to feed or to worry about. They were also not very familiar with the world, so their expectations were not that much high or didn't envy others too much.

Answer 2

Also, generally the slaveowners would not separate the parents (sell one off to another "master") until the children were weaned. This was more out of a selfish concern for "raising good and helathy stock" rather than compassion for the enslaved and their emotional ties to their families.