So you have a Love Triangle between Alice, Bob and Charles. However, the tension can't be sustained forever, so finally Alice and Bob confess their love and all is happy. Except for Charles; doesn't he deserve true love? Well, okay, but where can we pull out a love interest on short notice? Alice and Bob's baby! It would be like marrying the best qualities of both.
This Trope is about a love triangle being resolved by involving the offspring of the primary couple.
As this is a Love trope largely focused around endings, beware of spoilers.
"All spoilers will be unmarked ahead. Beware."
Not to be confused with developing the Love Triangle into a Love Dodecahedron. Subtrope of Love Father, Love Son. See also Unequal Pairing, which is likely in these circumstances.
The Yuri manga The Structural Formula of First Love by Shuninta Amano is about a lesbian whose high school love dumped her for a guy, but her daughter from him fell in love with the protagonist, in turn. It wasn't entirely intentional, however.
An old EC Comics science fiction story had an astronaut testing a new spaceship. He returns home to find that decades have passed, despite his only having experienced a short time away. His girlfriend, thinking he was dead, married his best friend, and they're both old now. But their daughter is now a beautiful young woman. The hero falls in love with her and they marry.
In a crossover story of Code Geass and Death Note, Lelouch mentions that Kallen isn't allowed to love him, Kallen believes that its becuase he is in love with C.C. Not to worry, after Kallen gets kidnapped, she falls in love with Mello.
In Sharknado, protagonist Fin is pursued by his younger employee, Nova, though he clearly is carrying a torch for his ex-wife, April. At the end he and April reconcile (a whole six hours after her boyfriend dies!), and while it's not clear, it seems Nova is going to hook up with their son Matt, who is closer to her age.
In Twilight, Bella and Edward end up together, but leave poor Jacob alone. Not to worry, when their daughter Renesmee is born he "imprints" upon her, essentially becoming romantically bonded for life with an infant.
In Anne McCaffrey's Talents books, the Rowan/Afra/Jeff triangle gets resolved in favour of Rowan/Jeff, and Afra is subsequently paired with their daughter Damia.
A very weird one occurs in Doctor Who. Amy has Rory and the Doctor as potential love interests, and chooses Rory. The Doctor has zero romantic interest in her, but he does fancy Rory just a little bit — enough to kiss him once, at least. (Rory, for his part, is just completely Amy-sexual.) Amy and Rory have a daughter, River, who grows up to marry the Doctor. It's severely complicated by the fact that while they are all time travellers, they are not all necessarily travelling in the same direction or at the same speed. The Doctor meets River before he meets Amy and Rory, on the day River dies, and likes her before anyone except River knows who her parents are. River decided, when she was a kid, to grow up together with her parents at the same age as them, never telling them who she was. Oh, and River has another mother — the Doctor's Bond Creature, eternal Love Interest and wife, the TARDIS.
Variation in the original V where Robin Maxwell loved Kyle Bates, but he ended up with her rapidly grown child Elizabeth instead (although there was never a triangle in the first generation).