Bitten by Baby, Forever Changed

User fell in love with a vampire named Isabella and married her on the condition that she would never turn him into one of her kind. Life was good; he got to keep both his love and his humanity. However, he forgot to account for one thing: their teething baby daughter, Luna. Her biting him for the first time brought about an unexpected change. "Could you stop laughing at me? This is serious," User pleaded, holding Luna at arm's length. "Sorry, sorry," Isabella replied, struggling to stifle her laughter. "It's just that all these years, you basically wouldn't even French kiss me, and now this happens! Ha ha! Okay, I'll admit I didn't know that actually rolling on the floor while laughing was a thing, especially for vampires. But here we are, holding our toddler at arm's length." User felt blood dripping down the back of his hand. The little rascal had the audacity to giggle at him as well. "All these years of caution for nothing," he muttered. "Okay, okay, I think I got it all out," Isabella said, finally composing herself. "Give her to me. You go get a tissue unless you want me to take care of your wound." She took the adorable little Luna from his hands and gave him her best deadpan look. Despite the occasional twitch of her lips and her still-trembling shoulders, she really pulled it off. "I didn't even know babies teeth this early," User grumbled, which set Isabella off again. "Look at the bright side," she said between laughs. "You'll be around for much longer now. No more sunburns. You’ll be able to literally bite your coworkers' heads off. And we can share a blood bag—it will be so romantic!" User chuckled at that. "You always know how to put a positive spin on things." "That's my job," Isabella replied, her eyes twinkling with amusement. "Welcome to the family, love. Eternity just got a lot more interesting."