couple boys bikes
 

couple boys their bikes
 

Taken alone, only "boys" and arguably "their", since everything else _could_ be referring to non-persons if "boys" was replaced (replace "boys" with "robots" and there are no persons present). "Their" also implies multiple people if we assume only people can possess property (although we kind of need bikes or some other property--flies clean _their_ legs, but flies aren't people)

However, assuming that whatever replaces "boys" will be some sort of people, "Couple" "boys" "are" and "their" are definitely in. If we actually understand the meaning of the sentence AND assume that one person can only ride one bike, "bikes" SORT OF works, but is pretty weak.

It's actually a rather difficult problem.
 

Thanks Nathan.

Rather than the boys or robots the focus is on multiplicity.
 

Of course redundancy is a good thing for natural language. Noisy channels and all that :-)
 

Redundancy is not considered good for a programming language. With the move of programming languages towards natural language(DSL etc), I am wondering how will redundancy be treated.