@kloverdarling I wouldn't worry about the chicks keeping warm. They get up under the mothers wings and burrow into her feathers when they are little. I've had broodies take 3 day olds out into 40 degree weather. The run around and when they get cold they go back to mom.
The nest needs to be big enough for all of them, so some of that depends on when they hatch. They won't need the heater, and it might keep mom too warm.
She may or may not get off the nest. Don't bug her too much (I know,easier said than done. My girls get tired of me fishing around under them feeling the eggs for pips sometimes).
Good luck and good hatch.