Liquidise the cabbage with hot water. Decant the liquid into a clean glass jar, using a coffee filter paper. Alternatively, boil the cabbage in water until a lot of pigment has gone into the liquid. Leave to cool and sieve.
The resulting coloured liquid is your indicator and should be labelled. It will last for a few days in the fridge.
Collect a selection of household items to test. You could use lemon juice, water, salt solution, washing powder solution, vinegar, baking soda solution, borax solution and so on. Make sure the chemicals you are using can be safely used by children, and don't mix them unless you know they will react safely.
Place 5ml of each of the items to be tested into a clear labelled glass.
Add 2ml of indicator to each and note what happens to the colour.
To make pH paper, soak filter paper in the cabbage solution, and hang it up to dry. Cut it into strips and use these to test the acidity of the chemicals. Acids will turn cabbage water redpink, alkalis greeny blue, and neutral solutions will be purple.
Your students could investigate which vegetable makes the best indicator, and then write a guide explaining how to use it, showing examples of what each colour would indicate.