You didn't say how old the children are, or just what you are supposed to be teaching. Are you emphasizing environmental relationships? the cycle of growth? plants to attract butterflies and or birds? or ?

Lots of perennials can be planted in the spring as well as in the fall. I recommend bluestone.com for reasonable prices and great service... their site is also very informative re soil requirements and bloom times. Also, most of their plants *will* bloom this year.

Why not plant annuals? The quickest to bloom from seed are marigolds and zinnias. Zinnias come in many sizes and colors and are very easy to grow. Also more tolerant of poor soils, so if you don't already have a nicely enriched planting bed, they will forgive you and bloom anyhow.

I highly recommend going to the 'new to gardening' forum and making use of the 'search' feature. Try keywords such as 'teach' 'children' and 'idea'.

I'm hoping you will have your garden in the sun. If you are planting seeds, how about the easy grow, drought tolerant plants, such as Sunflowers, Zinnias, and marigolds. Also easy grow vegetables, like Tomatos, Okra and Squash. Potato vine would be a nice touch. If time is a factor, you might want to plant established plants along with some seeds. If your wanting to plant perenials, there are others out there that can help you better than I can. Hope this helped a little.
Karen