It's the opposite, clear water freezes at a higher fusion point than salt water.
tell to your kid to talk about this:
http://en.wikipedia.org/wiki/Eutectic_system#Others
for the cinetic:
http://www.engineeringtoolbox.com/th...ity-d_429.html
http://en.wikipedia.org/wiki/Brine#Properties
the thermal conductivity is slightly the same, so there is no reason that it freezes faster at it's fusion point.
ps: this subject is probably harder to handle that you think it is.
http://en.wikipedia.org/wiki/Freezing_point_depression