Here are some ideas for you to try:
To determine which substances are an acid and which ones are an alkali. The cabbage water acts as an indicator, changing colour if it is in contact with an acid or an alkali. Even if the concept of acids and alkalis is too complex for your child, they will wonder at the amazing colour changes in this experiment.
The cabbage water should turn pink in acidic solutions, and blue or green in alkaline solutions.
Kids could also go on to make and try different indicators such as the juice from freshly boiled beetroot, blackberry juice or strawberry juice. They could also go on to use these natural colourings to dye some scrap materials.
To guess which egg will cook first.
Answer: Both eggs should be the same when you open them up, because the water boils at 100 degrees, so even if the one flame is hotter the water will not get any hotter!