I have a trio of OEGB chicks ( 7 weeks old ) and I wonder if they are old enough to have grit and greens in their diet? These are my first chickens since 1958. They have grow crumbles always in the pen with them but when I take them out in the yard ( daily ) they peck and pull leaves off the pole beans and tomato plants and seem to enjoy the leaves a lot. Also the roo managed to catch a grasshopper when it blundered in the cage. My problem is the local feed store only has grit in 50 lb bags and the chicks only will be in the cage about 2 weeks till I get the coop & run finished and predator proof. Can I go to the river and scoop up river sand & gravel and put in the pen in a cake pan for them? What size gravel/sand do they need ? I am guessing we are not talking pea gravel or larger?