That's fine. They don't need a lot just enough to use in crop to grind. Leaving too much out would only lead to them scratching it about and possible waste. If they've access to dirt they can find their own. I will gather small stones from side of road or driveway now and then in winter for mine as the run gets frozen over. What happens if they don't have enough is they eat more food.
I start the oyster shell application about time of first lay. My preferred method for application is to simply scatter it over grass but it can also be keeped in a small bowl where consumption is easy to monitor.
With the kid's petting zoo interest. consider using live mealworms. Use live over free-dried. You will see some very interesting interactions not realized by using scratch or feed.