There are so many different directions to go in if your child loves space or you are trying to foster an interest in astronomy. Let's start with observing.
Telescopes, binoculars, and observing gear. What children really need, more than "stuff," is for someone to take the time to stand outside with them on a starry night and point out planets and constellations. Teach them the sky. You can give coupons good for "1 Hour of Observing with Mom" or something similar. If you are looking for gear, start small. You don't want to make too much of an investment if this interest is just a flash in the pan. Binoculars are an excellent choice because they are easy to use and have uses outside of astronomy. If you really want to buy a telescope, stay away from the cheap and rickety ones that advertise outrageous views on the side of the box. Get realistic. For less than a hundred bucks, you are not going to see detail on the surface of Mars. You will see great detail on the moon, the moons of Jupiter, and the rings of Saturn, but beyond that be very wary of "awesome" observing pictures that look like they were taken with the Hubble Space Telescope. For a good entry level telescope, try Celestron's ExploraScope.
Games! This is the best category, because you can play them even when it's cloudy and 20 degrees outside. And the whole family can usually get involved.
Models. A large crop of solar system models allow for a wide range of gift giving. You can get the inflatable solar system, which is fun for setting up in the yard and trying to make it close to "scale" or for tossing around and getting some physical activity. Or you can go with a glow-in-the-dark, even friendly-for-babies mobile for the nursery or bedroom. Or how about the very cool motorized solar system model that replaces the light in the bedroom with the sun, as the planets circle around it? And don't forget about the moon. This moon functions as a night light and teaches the phases plus an audio tour guide.
"Toys." I say "toys" because while there are plenty of miniature sized space shuttles and Saturn V rockets and astronauts along with glow-in-the-dark star stickers, solar system puzzles and Mars or moon sand that can be picked up just about anywhere, why not try some really cool "toys" that are fun to play with for kids and adults and that encourage a lot of learning? With the Discovery Ultimate Star Planetarium you don't have to leave the house to see your own planetarium show or discover what the sky looks like Down Under. (But caution: see my frustrated review.) A similar but less expensive spin on this toy would be the Star Theater Home Planetarium. The 3D Space Projector comes with good quality 3D glasses to view solar system to deep space objects projected on your wall. The Planetary Challenge is a toy and a quiz game all in one.
Books. Starting at the youngest age, pick up There's No Place Like Space, a book based on the Cat in the Hat characters. Seymour Simon has written a great series of books on the planets, stars, and other space objects. The Big Dipper and You is an excellent book for children who want to learn the sky - it gives them an always-visible object to learn and branches out from there.
Software. Check out the all-encompassing The Eyewitness Encyclopeida of Space and the Universe or join Miss Frizzle and the gang in the Magic School Bus Explores the Solar System. For older kids or those looking for a deeper understanding of space, consider Starry Night Backyard.
The best tip for buying a space gift for your child is to find something YOU would be interested in. The more you interact with your child, the more they will be interested in the gift and in astronomy in general.