How were children treated before and during the Enlightenment?

For most of the eighteenth century (1700s) there was no concept of childhood in any recognizable modern sense. In other Words, children tended to be expected to pass straight from physical dependence to something close to adulthood.
The period of physical dependence was taken to last up to about age 7-10. After that most children were expected to work adult hours ... and if convicted of a crime were held fully responsible and punished as adults.
In Britain and France the 1770s saw the gradual rise of the concept of an intermediate stage between physical dependence (infancy) and adulthood, namely childhood. The first books written specifically for children and some children's clothing began to appear for the first time. This development was largely confined to the middle classes, and for the poor it had to wait until well after 1850. The concept of adolescence - that is, a period between childhood and adulthood - is even more recent.
I've deliberately avoided mentioning the Enlightenment, as the beginnings of the concept of childhood are generally associated with the Pre-Romantics.

For details see Philippe Aries, Centuries of Childhood, Penguin. (The book first came out in French around 1960, and some of it may have been superseded by more recent scholarship).