I would start by using the story Aliens wear underpants as a stimulus for discussing space and aliens.
We would discuss what we know about space and aliens.
I would ask what would an alien look like? We would draw or paint aliens, we would make our aliens using 3-d materials- clay/ construction materials/ playdough.
Then we would discuss and classify our aliens- 3 eyed aliens together, aliens with 3 heads together etc
We would then discuss the aliens home in space. We would create a small world area with constellations (stars) and the moon using concrete objects. Then we would play with our aliens in the space areas.
We would also create rockets and spaceships that we would use in the small world area.