Actually, you will not be there for the marathon days, Jan 7 and 8. As to the crowds: expect huge crowds the first three days. The holiday crowd does not start to leave until Tuesday and it does not really get lighter until Wednesday. Also, like you, there are many whose schools are out the entire week That is becoming more common for the break time these days and thus crowds will actually be somewhat large even Wednesday through Friday (that's how it was in 2011).
As to weather be prepared for anything. 2011 was good with day time temps usually in the 70s every day. 2010 was ... well it was something one tries to but cannot forget. Freezing everyday with highs in the 40s and lows in the evening and night in the 20s. There were even snow showers one day.