really at that age, any time is fine. We pulled our kids out until they got in the older grades. Last time was when our oldest was in middle school. At that point it was just too much. But for 1st and pre-K it's no problem.
If it were me, I would go end of January/beginning of February. Crowds are low, weather's nice, and at that point I'm ready for a break from winter!!! If not, November pre-Thanksgiving is good too.