Hmm. It's not going to be very practical to shuttle 15 chicks back and forth between indoors and outdoors, is it? I did it last year with five chicks, and that was enough of a handful.
You could try putting them out on a day when you can keep a close eye on them, checking every hour or so to see how they're doing. If they're panting, holding their wings out, and looking distressed, you can take steps to help them cool off.
I forgot to ask...are your chickens heat tolerant breeds? If they are, they probably will manage the change over ok with your close observation; I'm sure they'll appreciate the extra room to roam, too.
I have heat tolerant bantams. I've noticed that even when my hens are panting, the four and five week old chicks aren't bothered by the heat. They've been outside during the day since they hatched, but it seems like we went from "warm" to "OMG HOT!" very suddenly this year. Keep an eye on them and your chicks will probably manage the transition just fine.
They're lucky having someone so concerned about their well being taking care of them.