My cat is scared to death of the chicks, but loves to watch sparrows out the window. Lots of scolding seasoned Daisy against chicks.
But if your cats won't stand that and/or are rather wild, try either
1) covering the brooder when you're not using it
2) putting the chicks' brooder in a room with a door that is warm and that the cats don't like. Daisy, for instance, can't go into our guest room because my aunt who has cat allergies stays here sometimes. If you go "psssssssst!" feircly whenever they enter the room or squirt them with a water gun, they won't have fond memories of that place! Of couse, there's always the chance they'll just associate it with you and not like you anymore, but I cuddle Daisy and I'm her "mommy" (heck, the cat is almost three years old!!!) so she still loves me.