Is not having children exposing them to life's various pains and an unpredictable trial, so why doesn't one bear life's loneliness alone instead of bringing another into existence to suffer?

The core idea of the questioner clashes with the wisdom of existence, for Allah created humans to test them. And the test comes through religious rulings and divine decrees. The questioner's inquiry invalidates the divine wisdom in creating humans. Life contains both pains and blessings, and it is not pure evil, but rather, humankind is ungrateful, forgetting the blessings of their Lord. Let the wise person beware of a blameworthy view of life, and let them know that tribulations turn into blessings through faith. Allah rewards a good deed ten times over, so why would a person not hope for the rectification of their children so they may benefit themselves and their parents, and so that the destiny of all may be Paradise?