In the Maughold parish register the mother's name was hardly ever recorded at baptisms before about 1718 when her name started to appear occasionally, and still only appeared rarely until about 1728. There were about three William Chirmeen(e)/Kermeens etc. having children baptised in the late 1600s/ early 1700s, some identifiable by their land. So far as William Kermeen and Ann Christian were concerned, the eldest son was William bapt 1711 (Corony), recorded in wills and land records as the eldest son and heir. The eldest daughter Ann (Stole) died leaving young children, who were left in the care of their maternal aunts and uncles, all named.
If there were Maughold baptisms which were not recorded in the parish register, it isn't obvious - there are no large gaps like in Andreas, for example. On the other hand I have often seen previously unknown children named in wills.
Sue