Don't know how it would set to music or relate to elementary school children's ken, but:

Spring Pools

by Robert Frost

These Pools that, though in forests, still reflect
The total sky almost without defect,
And like the flowers beside them, chill and shiver,
Will like the flowers beside them soon be gone,
And yet not out by any brook or river,
But up by roots to bring dark foliage on.

These trees that have it in their pent-up buds
To darken nature and be summer woods--
Let them think twice before they use their powers
To blot out and drink up and sweep away
These flowery waters and these watery flowers
From snow that melted only yesterday.

©1963 by Robert Frost

(etaoin, a quick call to the Frost estate, if you're interested in using any of his material, is probably all that is required...they should be delighted to grant permission for his work to be used for such a project, pro bono...you just need their written permission, that's all)