I see. I would consider either:
1) setting up a temporary fenced area in the yard, somewhere in a shaded or partially shaded area, for at least a week or two, or
2) propping open the human door in the coop (if that helps cool things off inside) during the day and having the chicks in a fenced off area inside the coop, again for a week or two. If going with this option, definitely check on them regularly the first few days to make sure they're not overheating.
In either of the above, the chicks can be kept in a fenced off area, such as a dog crate, inside the coop at night. Or you'd need to bring them back to the brooder at night. I would not have them loose inside the coop until I knew they were safe enough with the older birds that they wouldn't be attacked or harassed.