If you can get a clump of grass/weeds with dirt from the yard, they will pick thru the dirt for small grit/rocks/pebbles. You can also give them a dirt box instead of sand, dirt from the yard (I put mine into one of those plastic sandwich type tupperware containers) But if you cannot get dirt, wait to give them anything needs chewing or grinding to eat (for example - eggs, hard boiled or scrambled are fine to feed without grit but grass, cabbage, apple, bugs need to be "chewed" or ground in the gizzard which needs grit to grind).
You can also put a stick or two into the brooder for them to jump around on and inspect. A stuffed animal, or different stuffed animal.