This year Little Man and Little Red are using Apologia Science's Exploring Creation with Astronomy.
In the first lesson they were instructed to make a model solar system. We used styrofoam balls, balloons and even beads to get a relative size of each of the planets.
I used this same book with Legs a few years ago. It is a text book but the information is presented in a story-telling format and broken down into reasonable amounts for each day. The assignments or projects are simple and use things found in most households.