I give mine 1/2 or less of a hard boiled yolk every now and then. I give them their "other" favorite treat almost every day though! The other favorite is a little Chick Starter (I do about 1 Tablespoon) mixed with some Hot Water (from the tap, but agaon, about 1 Tablespoon). They go equally crazy for both treats, but I give mine Chick Grit when they're going to have the yolk, just be on the safe side. They also have a sand box full of play sand that they thought was another treat, but after a few "cement poos", they've given up that habit.
ChickenCrazy - I would be careful about the egg shells...I've read on the site that giving a chick too much Calcium before they're laying eggs of their own can really screw up the development! Also, it isn't really a grit...they can "digest" it.