How old are you chicks? Do they have a regular feed aside from this treat you describe?
Bread is never ideal for birds as it's got energy but no nutritional value. Small amounts of milk are okay but chickens aren't made to digest dairy.
If I have milk that expired.. I set it out on the counter and let it curdle some (aka ferment like yogurt or buttermilk)... then pour over the top of the chicken feed... They do love it and nothing get's wasted!
It's important to keep treats below 10% of the total daily intake if possible so that nutrients stay balanced in order to have the best immune system response and health possible.