It varied a lot with time and place. At times, it was voluntary. Give up a son and be tax-free for up to seven years (although 4-5 seem to have been the norm). It was also resented and there are indications of families maiming or otherwise making their sons unfit for Janissary service.

Janissaries could and often would end up in service as civil servants, either directly (if they showed promise and were intelligent) or after their military service. The life as a Janissary meant (at least if you avoided or survived military service) a life of comfort and security, and often wealth and influence as well, which can explain why some even bribed officials to take their children.

The child was often between 7 and 10 years of age when taken in to the Janissaries, and could remember his family and provide them with help and influence should they need it later in his life.

Towards the end of the 17th century, the Janissaries had become a state in the state - they were allowed to marry and own land, secured state resources for themselves and often made sure thier children inherited their positions. The system was no longer restricted to christian children.

So, to make a rough answer to your questions.

Early on, it was widely resented and fasmilies were known to sometimes maim or harm their sons to make them inelegible for Janissary service. Children were taken by force, yes.

In some places and later, it became a good way to secure a career for one of the sons of the family, and get someone at least friendly to the family in high positions in the civil service or military, and it became so popular that som efamilies bribed local civil servants to select one of their own children. Sometimes muslim families would bribe officials to note them down as christians to have one of their children selected.

Even later, the Janissaries became a state within the state with de facto inherited status, land ownership and the right to marry (which the early Janissaries did not have).