A voorkind, literally a before-child, is a child born before the marriage.
In the 1900s, this term was commonly used for a child born out of wedlock, before the mother was married.
In the 1600s and 1700s, it could be used to describe the children of a previous marriage. For example, in a will, you may find a parent describing what to leave to the voorkinderen (children from a prior marriage) and what to leave to the nakinderen (after-children), of the current marriage.