Let your child lead in science while you steer from a distance. Suggest, don't take over. Also, until they are 9-10 years old, there is no real need to lay out the specific vocabulary for what is going on. Just say, "What do you think will happen?" "What should we try first?" Even if you know what the result will be, as long as they are learning and enjoying themselves, let it happen.
Compare brands of paper towels or paper plates.
Plug the tub and take a shower to see if a shower or bath uses more water.
Where in the house does bread mold the fastest?
On what material does ice melt fastest?
Is the inside of the car cooler with the windows only cracked, rolled part of the way down, or rolled all of the way down-does it really make a difference?
Set up rain guages in your yard and your nearby family or friend's yards to see if rain falls evenly in the same area.
Chart clouds to see if you can predict the weather.
Does salt water heat faster or stay warm longer than tapwater?
Test different fabrics to see how they insulate.
Do commercial rooting enzymes (or fertilizers) work?
Does hot water freeze faster than cold water?
Can you design a package that will keep a raw egg from breaking if you drop it from the roof or out an upstairs window?
Does a potted plant grow faster with worms in the soil?
Make and use a solar cooker.
Test the pH in your local water.