Botanicalbabe, not sure what age group you are teaching but beetroot (fresh) onion skins, mulberries, Gum bark or leaves would be a good place to start as these are readily available, the only danger would be the boiling process, fill a large stock pot with say onion skins and cover with water, a large pot is required and about 3/4 of an hour is required to boil out the colour, strain and cool. I put the dye into plastic bottles for when I want to dye the fabric. I dye about 1 to 2 mts of fabric at a time and you have to simmer this for about the same amount of time, this can vary but to test, you just check the water after a time as it will clear a the colour goes into the fabric. Set colour as I have already described and heat (iron) set when dry. All fabric was dyed with natural ingredients prior to modern dyes being developed , these were developed over time of course.
A good reference would be the tartan and plaid makers from the Isle of Sky (Scotland) who still work with traditional methods to dye wool using sea weeds and heather from the hills for the most beautiful colours imaginable. It is very time consuming but the results are to die for!!!
My Father was a modern textile dyer and textile chemist from Scotland who's training included the history of this craft although his work was with modern dyes in huge industrial dye plants dying wool for carpets and furnishings etc.
Gal