Massallah pertaining to a child born out of wedlock

The parents have always claimed the child to have been born premature, but he was not hospitalised in an incubator at birth, rather he was sent home. What is the parents responsibilty toward the congregation/s that have read behind such a person if they eventually do admit to themselves that the child is illegitamate?
NB. The parents made nikaah before the child was born.
Please Advise.
Jazaakallah