I love Circle Round!
When casting circe with kids, my best advice is to keep it simple and loose. Invoke the elements through movement, instead of poetry that won't hold their attention. Mark out the circle with sticks, leaves, rocks, cards - whatever strikes your fancy. This seems to help kids understand the boundary and not flit in and out.
Have fun! And remember that casting the circle can be a complete rite in itself, and doesn't need a lot embelishment.