A child obsessed with space is usually asking about something that is actually, physically there, every clear night, for free.
Cheap toy telescopes are often too blurry to show anything real, which is a quiet disappointment most children do not know how to name. A simple pair of binoculars, pointed at the moon on a clear night, reliably shows genuine detail and costs less than most toy telescopes.
Stuck to a bedroom ceiling, a set of glow-in-the-dark stars turns the last five minutes before sleep into a nightly ritual, which is more use than most gifts get in a year.
A meteor shower, a visible planet, or an eclipse happens on its own schedule and needs nothing from you except staying up a bit later together. Check what is actually visible from where you live before promising anything.
Usually not yet — they are harder to aim and focus than adults expect, and a frustrating first experience can put a child off before it has begun. Binoculars are a better first instrument.
A planetarium visit, if one exists nearby, is the honest substitute — it is real astronomy, even indoors.
Part of our guide to Gifts for one obsession.