<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _group_keys(self):
 """ every child key referencing a group that is not a dataframe """

        return [name for name, child in iteritems(self._children) if isinstance(child, GroupNode)]