As with any coop, you design the ventilation accordingly, no different with earth bags. As for a dog digging etc. The best foundation for any coop is rock. A shallow trench filled with rock under your earthbags allow for water to be diverted away from the structure, also having the rock...
Chickens need a ladder in the kiddie pool. Seems really odd since they can flutter up to their roosts and nesting boxes, but it's something about the water... Also, I would never have water deeper than 6". At that depth they can lay on the floor of the pool and still be head above water.