Disclaimer: I don't own Harry Potter.
"Happy birthday to you, happy birthday to you, happy birthday dear Luna, happy birthday to you," Pandora sang as she waltzed into Luna's room.
"Good morning, Mummy," said Luna with a yawn. She then held up three fingers. "I'm three today!"
"Yes, you are," her mother said proudly. "And remember how Mummy promised you'd get your very own pet?"
"Yes!"
"Well, today after your birthday party we'll all go to the pet store in Diagon Alley and let you pick out one." Immediately the toddler started to jump up and down and shriek; she continued to do this as she and her mother made their way down to the kitchen for breakfast.
"What's all this yelling about?" Xenophilius asked as he entered the kitchen.
"Oh, I promised Luna we'd go to the Magical Menagerie today and get her a pet," Pandora explained over her daughter's continued screeches. Xeno bent down and stroked Luna's hair, calming her down.
"Happy birthday, moonbeam," he said. "Are you looking forward to your party today?"
"Uh-huh! Can Cedric come?"
Her father took a moment to think about this. He hadn't wanted to invite Cedric to Luna's birthday party the previous year, but during the few times the two children had interacted, Cedric hadn't seemed that bad. Also, since it was Luna's birthday it wouldn't be fair to deny her what she wanted unless there was a legitimate reason to do so. And right now he could not think of any good reasons to say no.
"Of course he can come, moonbeam."
"Thank you, Daddy!" Luna said, hugging her father tightly.
It was a great party. Pandora had made Luna's favorite cake as usual, Luna and Cedric had played a lot of games including Pin-The-Horn-on-the-Snorkack, and Luna's favorite gift was a copy of Fantastic Beasts and Where to Find Them (for she had learned to read quite early). After Cedric had gone home, all three Lovegoods Flooed to Diagon Alley and then proceeded to the Magical Menagerie.
"Oooooooh!" the toddler squealed in wonder. The shop was full of so many kinds of animals: newts, rats, puffskeins, cats, toads, ravens, you name it. Luna walked to the section where the kittens were, taking time to look over each one before finally picking out a calico kitten.
"Is that the one you want, Luna?" Pandora asked. The little girl nodded, and they went to make their purchase, also buying cat food, cat litter, and some other supplies. Once they had arrived home, Luna started giving her new kitten a tour of the Rook.
"...and here's my room," she finished. She then picked up her stuffed Crumple-Horned Snorkack. "This is Hans. Don't scratch his horn, he doesn't like that. Here's Twinkles" - she held up her stuffed unicorn - "and this is Sunny." She held up her stuffed eagle. The kitten rubbed itself against Luna's leg and purred, and she reached down and petted him. "You like it here, don't you?" The kitten purred some more.
Later, as Luna was reading her new Fantastic Beasts book, she suddenly figured out what she wanted to name her kitten. She turned to him and said, "I'm going to call you Newt!"