Jacinta and her brother were a genuine test of the machinery, and the question they raised had been open long before them.
The problem
A canonisation that is not for martyrdom requires proof of heroic virtue: not merely goodness, but the deliberate and sustained practice of virtue at a level beyond the ordinary. The objection to the causes of young children is that a young child cannot be shown to have exercised that kind of settled deliberation, because a young child has not yet formed the capacity the standard is measuring. The objection is not a slight on the child. It is a claim about what the evidence can carry.
How it was answered
The answer that prevailed is that the capacity is not fixed by age and can be demonstrated in a particular child by what that child did under conditions that would test an adult. In this case the instances argued were the refusal to retract under the arrest of August 1917, and the acceptance of a fatal illness and of hospital treatment far from home.
The dates
Both children were beatified by John Paul II at Fatima on 13 May 2000 and canonised by Francis at Fatima on 13 May 2017, the centenary of the first apparition. The miracle accepted for the canonisation concerned the recovery of a Brazilian child after a fall.
What was not decided
The canonisation says nothing about whether the apparitions occurred. The two questions are separate, and the Catholic Church has never made belief in Fatima a requirement of anything.