I gave quite a few to my 12-day-olds this week and they had a blast. The first time was actually quite hilarious. I set four down in the brooder and they didn't even notice at first, and kept walking all over them, while looking up at me to see what I wanted. Finally one of them spied movement out of the corner of her eye and literally jumped backward, with this look of "OMG - what the heck is that?" Then some instinct took over and she stepped forward again, grabbed it, and raced off around the brooder giving the little "I got it, I got it" yell, while looking for somewhere to hide and eat her prize. The others meanwhile, couldn't figure out what the fuss was and watched this performance with great interest.
No ill effects from their treat. Keep in mind that Mama Hen will have them eating this stuff as soon as they're hatched! If you're concerned about grit, you can always put a little sand somewhere in the brooder that they can use if they want to.