Lots of different advice, so here's my 2 cents. Start weaning them off of the heat lamp...at least for daytime use.
Mine are the same age as yours (although mine aren't silkies), and when it's cold out (we're having highs in the 40s, lows in the 20s), I can feel them shivering a bit when I'm snuggling one on my lap, so I know how that makes you feel protective. It would be harmful IMO just to say "No more heatlamp for you!!" (plus you'd sound like the soup nazi). But I'd definitely start moving it further away, or reducing the wattage until they're okay during the day and you're just using it at night or on REALLY cold days. At least for this winter, I will continue to use my lamp when it drops below freezing (despite the fact that some say it's harmful to "coddle" them so). Next winter, when they're full grown, it may or may not be another story... I am curious as to why they're in your garage rather than a coop???