That's great to get your daughter involved!!! Check out a Randall's Goby and Pistol Shrimp for a cool nano pair!
Bleach will cut through the organics for you. Use about 1 cup per gallons. Let it circulate for 8-12 hours and check to see if it's done the trick. If not, leave it awhile longer. Once you're done, empty, rinse the rocks in your laundry tub, then refill with RO and circulate again. The remaining chlorine will off-gas. If you're concerned, add some Prime or AmQuel. Once you're satisfied salt it up, add some bacteria, and start "ghost feeding" a few fish pellets a day to kick off a cycle.