1. Police brings thief across the river
2. Police returns
3. Police brings boy across the river
4. Police returns with thief
5. Father brings (second) boy across the river
6. Father returns
7. Mother and Father cross the river
8. Mother returns Father and sons are on the opposite bank.
   
   
9. Police brings thief across the river
10. Father returns
11. Mother and Father cross the river
12. Mother returns Only Mother and daughters are left.
    
    
13. Mother brings daughter across the river
14. Police and thief return
15. Police brings (second) daughter across the river
16. Police returns
17. Police brings thief across the river Yippee! Job done.

There is another possible solution ... Go figure it out!