It took me around 12 minutes to figure it out, but here is the solution-
1. First the policeman and criminal cross the stream.
2. The policeman comes back alone
3. Second, the policeman and one of the daughters cross the river.
4. The policeman comes back with the criminal
5. Third, the mother and the other daughter go across the stream.
6. The mother returns alone
7. Fourth, the mother crosses the stream with the father. She stays there.
8. The father returns alone
9. Fifth, the prisoner and the policeman go across.
10. The mother returns in the boat alone.
11. Sixth, the mother and father go across the stream again.
12. The father returns in the boat alone.
13. Next, the father and one of the sons goes across. They both stay there.
14. The prisoner and policeman return in the boat.
15. Lastly, the policeman takes one of the boys across. He leaves him with his family and returns to pick up the prisoner.
16. The policeman and the prisoner cross the stream. Game Over!!
There may be other solutions to this puzzle too. Let's see if someone can do it in fewer moves ???