It could be an orphaned wizard child from a family that was killed by Death Eaters. This would give Snape an added sense of responsibility as he might feel guilty for being associated with the Death Eaters in the past.
Well, perhaps the plot could involve Snape finding an orphaned child in the wizarding world. He might initially be hesitant but then grow attached as he sees the child's potential. The story could also include Snape teaching the child magic, protecting the child from dark forces, and the two of them forming a unique bond that changes Snape's cold exterior over time.
The storyline could involve Rarity introducing the human child to the world of Equestria. She might teach the child about fashion and elegance, as that's her forte. The child could also learn valuable lessons from Rarity's friends, like loyalty from Rainbow Dash or kindness from Fluttershy.
Maybe a Pikachu adopts a human child. The Pikachu is part of a group of Pokemon that live near a human village. The child was abandoned, and the Pikachu takes it in. They go on adventures together, with the Pikachu protecting the child from other wild Pokemon and teaching it how to communicate with different Pokemon species.
Squirtle. Squirtle is cute and friendly. If it adopted a human child, it could show the child the underwater world of Pokemon. The Squirtle could teach the child how to swim, and they could explore underwater caves together. It would be a really fun and light - hearted story with Squirtle as the main Pokemon.
Rarity would use her natural grace and poise to handle parenting. She'd start by dressing the child in the most stylish outfits, of course. And she'd likely be very strict about manners, as she values propriety.