You can get a parent id from a child entity's key, but you have to get the parent entity from the datastore.
You can store a department name in every employee entity, which will save you one get operation at the expense of extra data and more complex code (you have to account for situations like a change in a department name).
A better solution is to keep department entities in memcache. This way you save a trip to the datastore and keep your data model and code simple.