Answer: I'm guessing you want some advice about what to plant. If you need advice about preparing the soil or other gardening tasks, you'll find lots of information in our Article and Q&A Libraries. You might also enjoy the Kidsgardening Web site at http://www.kidsgardening.com
With such a long, narrow garden strip, you can plant one larger rainbow shape or several smaller ones. Or simply have rainbow stripes in long straight lines...whatever you choose I'm sure the kids will be thrilled by the colors and the fact that they helped create the beautiful garden.
Red: salvia 'Flare'
Yellow: dwarf sunflowers
Green: bells of Ireland (or a foliage plant like basil)
Blue: cornflower 'Blue Boy'
Indigo: larkspur (dark blue variety)
Violet: purple asters
The blues and violets are probably the toughest, since the contrast between the colors is less stark.
Hope this helps!
Q&A Library Searching Tips