Are you just considering options if the eggs don't hatch? Because once you put the chicks under her she will most likely abandon her sitting duties shortly after and start tending to the chicks.
She will be fine sitting on the day olds. Slip them in at dark and be sure they are as close to day old as possible. She will most likely accept them as her own but it would be good to check on them first thing in the morning. Time it so you are able to do that.
My Topsy, a small Silkie, did just fine with hatching and mothering her two silkie chicks on her first attempt at being broody. My Smokey, a small Araucana, has hatched both LF and bantam chicks and she did just fine covering them all.
I wouldn't worry about your bantam covering her chicks unless you start putting a bunch of LF chicks under her. It's not so much when they are day olds that you would have problems. It's more when they are three or four weeks old and still trying to get under her for warmth, especially in winter.
. Houdinni had slided over so the chick could get out (I'm guessing) and after it was out, forgot about the other two eggs. IT was a bantam egg and I think another light brown egg. The bantam egg had a hole about a cm. big, and cracks all around it. The bantam is yellow looking 
