Planned For attribute to use child iterations.

We are considering an approach where we have a Timeline as 1Q2013 under which we have multiple iterations, say I1, I2, etc. These iterations are required to have multiple child iterations as the scope of the iterations immediately under 1Q2013 is broad. For example, I1a, I2a, I2b, etc. When I create a WorkItem, I am unable to see I1a, I2a, I2b, etc. for PlannedFor attribute. Is this a known behaviour ? How do I assign a WorkItem to such nested Iterations ?

My end goal is to be able to generate a plan at the level of such nested iterations.