The parent/child knowledge issue sometimes is worth violating IMO -- again in conflict with standard OO. This may not have any bearing on this particular problem but one instance where I found it fits perfectly:

• I have a set of file classes all derived from a common base class. Each file class represents a file accessed via some different transport (e.g., ftp, http, rsh, ssh).
• Each file class in turn has a directory class. A directory is just a special type of that file, so it has all the file methods plus some add-ons only directories can do.
• At construction time, all the caller knows is the file name. It's during construction, when the actual filesystem (or whatever) is consulted that knowledge of a directory arises.
• If a file does find out it's a directory during construction, it simply morphs itself into its directory subclass by re-blessing itself into that class.

I like Perl's ability to elegantly address that pattern when it makes sense.

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