So many attempts to be helpful, but the best bit of advice is from
@TheDawg . Find a heating pad. Find something furry (not made of yarn or they can choke on the fuzz) to attach to the bottom side of the heating pad to simulate the warm fluffy underside of a broody hen. No need to move the chicks other than to make it safe for them.
The heating pad should be raised up and shaped so it's high enough for the chicks to crawl under while still touching their backs so they can directly contact the heat. Forget trying to make a 90 degree brooder. In another week, the chicks won't need heat during the day because they will have enough feathers to conserve body heat as long as they consume calories. At night they will require heat until age five or six weeks when they will wean themselves off heat.
Leaving the chicks in with the other chickens will reserve their status as flock members, eliminating the need for integration at some point.