I would check with your 4-H extension office for one thing. They have a plethora of (usually) free resources that are great for all ages. For instance, they have great nature education programs, something that is appealing to young kids.
I used 4-H with my two older kids. We would pick one of their workbooks and after the kids finished it they would actually receive recognition from 4-H for completing the program. One year we studied the trees in our area. Just very easy stuff to do like walking around the neighborhood or local park and identifying trees.
Your kids are young so make it easy on yourself. Forget about textbooks for science, they are a waste, especially at this age. Buy some blank books, such as the ones described here:
http://www.barebooks.com/books.htm
and have your kids fill them with pictures they draw of nature. Leaves, birds, insects...whatever you can think of. Collect rocks and leaves.
You see how you can "do science" with young ones very cheaply and have a lot more fun than going through a dry textbook.