you can get tees at:
The Dollar Store
Wal-Mart
Michaels (2 for $5 sales sometimes)
other retail establishments (like Kohl's)
online at:
Jiffyshirts
shirtsupplier
to name a couple. I've purchased tees at all these places! I found the return policy for jiffyshirts to be more prohibitive than the one at shirtsupplier. Also found little pinholes in the jiffyshirts that I didn't notice right away but when doing the final iron, there they were!
I can't answer Dec. & long vs. short other than to suggest you try a weather web site and get the avg. highs and lows for Orlando in December then let that be your guide based on your kids tolerance to the avg. temps. And hope for an avg. time!