Are you able to move the heat lamp outside too?
If so, you can move them and the heat lamp out to a bigger pen, any time you want (make sure the pen has protection from rain, predators, and excessive sun.)
If you are not able to move the heat lamp outside, try leaving them in their current brooder tonight with the lamp unplugged. Check on them a few times, and see how they do. Of course if they get too cold you would plug the heat lamp back in, but they might huddle up and be fine--and the easiest way to find out is to try it. If they do fine indoors with no heat lamp, then consider how much colder it will be outdoors, and decide when to try putting them out.
You could also carry them outside for each day, and bring them back in at night, until they get a bit bigger. It can be a real bother to catch them and move them twice a day, especially for that many, but it is one way to provide more shelter and maybe heat at night, but give them more space in the daytime.