After marriage, Undyne and Alphys settled in, and Alphys bore a son. Undyne was a little iffy about a new child coming, ecspecially after becoming the head of the guard again, since they were above ground, they reckoned they could do without the guard, only to come to realize, humans had not accepted them yet, and some were still trying to fight against them, so the royal guard started back up, under Asgore, and the new queen Alaxander's rule. Undyne never expected she would have a real child, of her own, little could she picture even though Alphys was pregnant.
Nine months later, Alphys had the child, and Undyne couldn't help but go head over heals for this new monster boy. He was the most precious thing ever. She rocked him gently in her arms, Alphys gasping at how gentle Undyne was being with her prince, Alphys would have expected her to shake him so hard he'd have shaking baby syndrome, but nope, she couldn't have been more gentle, maybe Undyne wanted to hold off the rough-housing until their son was older. It was her child, she had a child... And now it was her duty to protect him.
"I'll call him Neptune." Undyne chuckled, rubbing her newborn son on the chin, "And we'll be best friend."
"N-Neptune?" Alphys said, sitting up in her white robe, hospital food in front of her.
"Yeah, after my father," Undyne said, smiling, "He was a wimpy loser with a big heart."