I don't brood chicks indoors, ever. The amount of dust that many birds would produce......staggering.
If your greenhouse is large enough, I'd set the brooder up out there. Portion off an area and set up your heat lamp, leaving plenty of room to let them get out from under the heat. Have a couple of feeders and waterers so everyone can get access.
It's really interesting to watch a broody hen raise chicks. We think they're so fragile and need to be kept so warm, etc. By 2 weeks my broody raised babies spend hours at a time out from under momma, even in the early spring. And my best broody calls it quits at almost exactly 6 weeks, it's like she read that's how long she's supposed to brood them lol. At that time they weren't really able to fit under her anyway, so it was more for comfort they'd huddle together. Plus, the chicks do huddle and keep each other warm, like kittens or puppies.
I prefer to have hardier birds so I take the heat away as soon as I can. If you're worried you could put a lamp out until 8 weeks or so, but really by then they're as feathered as they're going to get.