Your child is curious about science fiction but a wall of aliens and spaceships and made-up worlds can feel like a lot all at once. Start here. Each book builds on the last, beginning with a friendly robot and a wish-machine grandpa and climbing, one small step at a time, toward real SF ideas about space, time, and what-if.
What climbs across this trail is how strange and how demanding the science fiction gets. Start here means exactly that: the first book is barely science fiction at all, just a robot learning to be a mother, and each step adds a little more, near-future detail, a dying city, travel through space and time, until the last book hands your child a whole invented world to weigh. There's no rush and no wrong door: a child can jump in wherever the strangeness still feels fun, and the whole trail is built to stay inviting the whole way rather than graduate into gloom.