That's a really tough spec.  (I can give a perfect - if edgy - suggestion for the equivallent mother and daughter, but father and son is more awkward.)
This list gives some possibilities. 
Ignore the first one (on the grounds of set). 
The second item is a set of sketches and essentially trivial (and it sounds like you want something with more beef).
The third play might be possible, but you may not like the subject matter!
The next three are Sherlock Holmes stories; you would have to have a relatively young Dr. Watson.
The final one is only understandable if you are a fan of British TV Science Fiction.

(As I said, a difficult spec!)