Although textbooks seem like the easy way to go for parents, it's not always the best way to help children learn. The more senses a student uses, the more they will remember what they learned.
After reading picture books for several years, I noticed how much we were learning about Science and Social Studies. I started collecting the lists of books that we were using and put them into categories. One year we might focus on animals and plants. Another year we might focus on weather, water, and the ocean. There are not all that many areas of science to cover, so it's pretty easy to pick some topics and books and have a great year.
As children get older, they can explore the categories of science in greater depth, but at first parents can just touch on a couple categories and use picture books. The picture books can spur even more learning because if both the parent and student know the subject matter they are learning, they can find opportunities to learn all around them. They might read several books about weather which might cause them to start a weather chart, watch the clouds one day, talk about the water cycle, watch a program about weather, and so on.
I think this method of learning science with picture books can promote even better learning and retention of material than any textbook or workbook every could. For a complete list of the books and categories I use, just check out Homeschool Science and get all the information there that you need.