hi there,
didnt see the solution yet, but here is my answer:
Lets give name to everyone.
Mother : M
Father : F
Girl : g
Boy : b
Policeman : P
Prisnor : p
1. first F & s go to the other side
M,g , P , p ------------------> F , s
2. F comes back
M, g , P , p, F <-------------- s
3. M & F go together to the other side
g , P , p -----------> M , F , s
4. M comes back
M , g , P , p <--------------- F , s
5. P and p cross the river
M , g -------------> P , p , F , s
6. F comes back
M , g , F <--------------- P , p , s
7. M & F will go now
g ---------------- > P , p , s, M , F
8. M comes back
M , g <----------- P , p , F, s
9. and that was easy, M & g cross the river and everyone is through
--------> P , p , F, s , M , g
:-)