I would just go outside to a sandy pebbly spot and scoop a few spoonfuls of that dirt and put it in a dish in the brooder. They'll eat as much as they need and the sizes they need and scratch around in the rest. My guess would be that your chickies aren't really eating all the grit- they are likely scratching a lot of it out on the floor. Grit lasts for a while in the gizzard, so I don't think they need grit with every single meal.