Rather than cinder blocks, you might want to use the 12" x 12" x 2" patio blocks. You'll have to sink them flush with the dirt -- maybe leave them a bit high, since I'm not really sure how stable that dirt might be.

A couple of other points:

=> It's common to put a cheap tarp under the pool, and cut it to fit. That might help stabilize the dirt IF you make sure to cut off all the extra, so the tarp does NOT channel rain water under the pool!

=> You want to make SURE that rain water does not run around the circle. Once the pool is up, you may want to mound the dirt along the upper edge, so rain water is channeled around the pool rather than under it.