I'd be more tempted to put the babes from the brooder under mama than vice versa. It's much harder to create the right conditions for the babies in a brooder than just letting the mama do what she knows how to do. I slipped a couple of day-olds under my broody in August, and she's still taking care of them in the 40 degree rain today (they don't need warmth from her anymore, just protection from Artemis the bully hen.)