Opera seria works that have mothers of adult children as characters:
Carlo il Calvo (Giuditta);
Farnace (Berenice);
La verità in cimento (notably two of them, Damira and Rustena);
Agrippina!
Graun's Britannico (Agrippina);
Il Medo (Medea);
Griselda;
Giulio Cesare in Egitto (Cornelia; historically she only has stepchildren, but the opera never makes the distinction and treats Sextus as her own son);
Graun's Cleopatra e Cesare (Cornelia, by the same token as above; Gnaeus is also treated as her own son);
Ambleto (Gerilda, i.e. local version of Hamlet's mother Gertrude; notably a controversial if not self-contradictory character; I have a feeling that the early productions and maybe even the librettist did not know what to make of her);
Catone il giovane by Giambattista Neri (Cato's wife Lepida);
Graun's Silla (Fulvia - not that Fulvia - is there but she is not meaningfully contributing to the plot).
This list is incomplete, feel free to add.