The only child I know of is Jean, the only one that was sent in Canada with the Carignan-Salières regiment in 1665.
However, we can presume that Ferdinand de Gerlaise had at least an older son, since he was a "seigneur" or a landowner, and at the time, the oldest son stayed with the land as he was the one that inherited from the father with the charge of taking care of the rest of the family (and become a "Pater familias"). Younger sons was often sent to the army or something of the kind to augment, for the rest of the family, the patrimony of the family or to marry and start a new family of his own (and discharging the previous family).
You can maybe do research on Ferdinand Jacquet (which was is name, "De Gerlaise" was the land that he owned (meaning "from Gerlaise". Since he could own more than one land, he could be seigneur of more than one place (like "Hannetaux").
However, I guess it will be less easier than following tracks in Quebec where they kept good records, since the Liège area was the site of numerous battles afterwards. For myself, I am still looking but found nothing yet. If you find something, let me know, I will appreciate greatly.
I have much more information on Jean Jacquet de Gerlaise, the first one coming in New-france, who was the first settler in Manereuil Seigneurie (today Louiseville, near Trois-Rivières)and the ancestors of all Desjarlais of North America.
I Hope this helps a little bit!