Hey Nicole... honestly it's so much fun with kids... because they can collect what they want to use, make the dye, dye the fabric, then make (or wear) something from the fabric.
The colours are all quite natural... but you can get quite a strong pinky/red from avocado stones! Or maybe try blackberries for a darker purple colour...
Dye length depends on if you use a mordant (pre-soak fabric in soya
milk, for instance) and how long you dye for (can leave fabric in dye bath for 24 hrs!) ...
Rebecca's book is great - guides you through all the steps really easily.
Do share any results you get - I would love to see them :)