If you are going to sneak them in, it's best done at night so she will stay on them all night, listening to and becoming familiar with their little peeps and movements and the chick she already has can accept them too. Then their new mum can wake in the morning and think what a clever girl she's been! As mentioned though, you need to be there when they wake to ensure she has accepted them and doesn't attack as it will take no more than a few seconds for her to kill them if she feels so inclined.....and eating or part eating them would not be out of the question for her either.