Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
We must believe that Allah, the Most High, is Wise in all His actions and decrees, and that everything He decrees is based on wisdom, whether we know that wisdom or remain unaware of its details. Our inability to know the wisdom behind a particular matter does not mean that there is no wisdom in it, just as our ignorance of how certain things occur does not mean that we should deny their reality.
The suffering experienced by children is no exception to this principle. Rather, it involves great wisdom, some of which may become apparent to a person while other aspects remain hidden. Among these are:
Pain may be an indication of an illness or condition affecting the child, alerting the family to the child's need for treatment.
Some forms of pain have physical benefits; a child's crying and screaming when experiencing pain may result in benefits related to bodily functions, even though the details of this may be hidden from many people.
A child's suffering may become a cause for the repentance and reform of the parents; it may prompt them to reconsider themselves, abandon some sins and shortcomings in their duties, and return to Allah, the Most High.
These sufferings remind a person of the Hereafter; this world is not the abode of complete bliss. Rather, it is a place of trial, while Paradise is the abode of bliss untouched by pain, and Hell is the abode of punishment. This encourages the believer to work for the Hereafter and strive for what brings him closer to Paradise and farther from Hell.
Pain is one of the inherent aspects of worldly life; from childhood, a person experiences hunger, thirst, cold, heat, fatigue, and illness. This is the nature of the life in which Allah created human beings for the purpose of trial and testing.
It should not be said: Why did Allah not create children in a world in which they would never know pain? This is because the nature of this world is based upon trial. Had it contained no pain or hardship, it would have been a different kind of existence from the one Allah, the Most High, intended with wisdom.
Likewise, the coexistence of pleasure and pain in this world indicates that it is not the abode of final recompense. Among the evident wisdoms is that a person should long for an abode devoted entirely to bliss, namely Paradise, and an abode devoted entirely to punishment, namely Hell.
If a person is unable to understand the specific wisdom behind a particular child's suffering, he should not make his ignorance of that wisdom an objection to the wisdom of Allah. Rather, he should be certain that Allah, the Most High, does nothing in vain and wrongs no one, and that behind His decrees lie wisdoms and benefits, some of which a person may know while much of them remain unknown to him.
And Allah, the Most High, knows best.