Fairies already have the Green Thumb trait, and they can use Bloom to make plants progress to harvestable much more quickly, and also seems to increase its quality (from "Normal" to "Nice," and so forth), so if your family has a garden, the fairy can take care of it. If they are Handy, they can Fairy-fix broken items, which seems to take less time than using a regular repair. And the fairy-house is portable, so they can take it when adventuring in tombs; it's smaller than a tent so has more places it can be set; they complete the challenge and get the Ancient Coins and the higher Visa Level more quickly. But I believe gardening is the main advantage. I didn't know they can't cast auras on children - that's unfortunate. Can children ask for fairy dust? (I don't play kids, never have, except briefly in Sims 1.)