The question here is not so much about using “the peerage.com” as a source reference but being able to acknowledge the offspring of an affair and in particular acknowledge the mother.
Especially when the mother did not go on and become a spouse, or de facto.
The WikiTree does not seem to be able to make such a indication directly.
WikiTree seems to be trying to set itself above the standards of all other trees out there but this inability to make this distinction, to me, seems like a glaring omission.