The good people at Super Teacher Worksheets provide a lot of outstanding material for children to improve their reading and writing. The site was created by Tim Weibel, a former school teacher from Buffalo, New York. His worksheets are used by elementary school teachers all across the United States, as well as scores of EFL teachers around the world. The reason is simple: high quality, ease of use, and effectiveness with children.
There is an extensive science section (as well as other subjects) with an array of topics. My personal favorite is their collection of material about the solar system. The worksheet below is about the planet Mercury and comes with a reading passage, comprehension questions, and answer key. You can download it here for free. There are also articles about the other planets, the Earth, the moon and sun, dwarf planets, constellations, as well as projects and other activities.