Why the accusative here? I think the nominative is called for. Optimum/maximum dōnum est pater/parēns esse.optimum/maximum omnium donum est patrem/parentem esse.
So in an infinitival clause headed by a copula and serving as the subject or complement of a conjugated copula, the nested copula suddenly stops acting like a copula and instead acts like a transitive verb? Weird! I mean, I know language isn't supposed to be entirely logical and consistent, but even by linguistic standards, this strikes me as really odd (and interesting).Maybe I should add that infinitive constructions usually stand in the AcI in Latin (unless they are part of an NcI ... and with the exception of some examples from poetry).
Confer this sentence from Cicero:
Non esse cupidum pecunia est, non esse emacem vectigal est; contentum vero suis rebus esse maximae sunt certissimaeque divitiae.
from Paradoxa Stoicorum, 51.