Just for reassurance, I've got 5-day-old chicks outside in my outdoor brooder with expected temperatures in the 20's tonight.
They've got their plate, a generous pile of deep shavings, and the Big Red Bulb keeping the area around the plate well above freezing.
The guidelines aren't as well-agreed-upon for chicks as they are for adults, but what's usually recommended is as follows:
1/2 square foot per chick for the first couple weeks,
1 square foot up to 4-6 weeks,
2 square feet up to 8-10 weeks,
after that the full adult 4 square feet.
I have a vastly-oversized brooder -- 4x8 -- and they use the entire thing from about 2 weeks old.