2 PM - 10 PM supervision @ $8 / hour = $576
10 PM - 8 AM supervision @ $2 / hour = $180
8 AM - 2 PM supervision @ $3 / hour = $162
In effect, their individual time is worth $4 / hour during the peak hours and $1 - 1.50 / hour during non-peak. (At least in my concept of the busy hours in my family. I'm not sure what your family's busy hours are.)
I essentially agree with garyfan and Baron.
Edit: I double counted an hour somewhere in here. Dumb math. and fixed.