Don't know really - they never seem to quite get finished in this house. The one with the potato clock was used for a while. Although it was very simple. We also had a reasonable one where you had to build circuits to make lights and buzzers go.
Or not buy a kit and use stuff in the house. Look up BBC Bitesize key stage 2 science or the Science Museum website or You tube for ideas. You'll find alot of ideas for solids and liqids, density, gravity, friction etc.