Let:
h=human
b=big monkey
s=small monkey

1.  Row b and s over ( hhhs / bs )
2.  Row b back ( hhhbs / s )
3.  Row b and s over ( hhh / bss )
4.  Row b back ( hhhb / ss )
5.  Row h and h over ( hb / hhss )
6.  Row h and s back ( hhbs / hs )
7.  Row h and b over ( hs / hhbs ) 
8.  Row h and s back ( hhss / hb ) 
9.  Row h and h over ( ss / hhhb )
10. Row b back ( bss / hhh )
11. Row b and s over ( s / hhhbs ) 
12. Row b back ( bs / hhhs )
13. Row b and s over (  / hhhbss )

Here is a remark on this problem I received from Rob Pratt, Department of Mathematics of The University of North Carolina at Chapel Hill, "There are exactly three other solutions of the same length, and none shorter. It probably doesn't make any sense to explicitly list them all, but you might mention that there are exactly four shortest solutions."

I got this one from a Usenet archive on math problems which can be accessed here, look under logic problems, part 2.

Michael Shackleford, A.S.A.