For youngers, we pick a theme each year and I just buy books to support the topic.
Year 1: biology, plants, animals, human body, first aid
Year 2: earth sciences, weather, astronomy, geology
Year 3: chemistry
Year 4: physical science
If you have little kids, seriously though, don't stress over science. Retainment is minimal at that age.
With my middle schooler I've switched to Memoria Press science, and will stick with them as they develop the rest of the program for high school.