Solution to Japanese River IQ Game

[FinanceLoans]

I completed this in 17 steps: Step 01: Cop takes thief. Step 02: Cop returns. Step 03: Cop takes one son. Step 04: Cop returns with thief. Step 05: Dad takes other son. Step 06: Dad returns. Step 07: Mom takes Dad. Step 08: Mom returns. Step 09: Cop takes thief. Step 10: Dad returns. Step 11: Mom takes Dad. Step 12: Mom returns. Step 13: Mom takes one daughter. Step 14: Cop returns with thief. Step 15: Cop brings other daughter. Step 16: Cop returns. Step 17: Cop brings thief.

[SolarPowerPlant13]

I just don't understand, why the parents beat up their own kids, and (I know they're testing your IQ but) why would they bring a criminal? Is she their other daughter or something?