According to reports in the Belgian media (Wednesday), Pope Francis promised them new compensation during his visit in September.
Victims' lawyer Walter Van Steenbrugge said: "The Pope considered the amounts of 2,500 to 25,000 euros that had previously been paid out via the arbitration tribunal to be too low in view of the immense suffering and damage."
However, the Vatican has not yet responded.
According to Van Steenbrugge, an initial letter was sent on 7 October via the organisation Human Rights in the Church, which brings together those affected by abuse.
It was addressed to the nuncio in Belgium, Archbishop Franco Coppola.
By bailiff
As he did not respond, a reminder was sent in Italian translation at the end of November.
According to the information provided, a third was finally delivered by bailiff on 26 December.
According to the lawyer, there is now proof that the letter was received.
Pope Francis visited Belgium and Luxembourg from 26 to 29 September.
In the capital Brussels, he met 17 victims of sexual violence by Catholic church representatives and spoke with them for two hours.
The meeting took place behind closed doors and was considered one of the key events of the trip.
According to lawyer Van Steenburg, four participants said: "The Pope apologised to them several times."