I found a few clearance books re: science fair projects using household materials when she was about five. We used them for years. They were a great jumping off point for additional variable manipulation and data collection, since the bare bones of the experimental design was already laid out for us.
Be aware, though, that this works best with parents that know a LOT of the theory behind why certain things work (and why some of the experiments/demos do NOT work as they are 'supposed' to). We had a lot of flexibility in improving design and improvising materials.
