Here's what I've been doing for years, and it works very well. Just go outside and pull up a clump of grass or dandelion, etc. with its roots and attached dirt. Put the whole thing, dirt and all, into the brooder with the baby chicks. They will be able to get a sufficient amount of the grit proper to their size from that. I have a brooder with four day olds in it right now, and gave them a generous heap of clumps last night. It's fun to listen to them "talk" about it as they investigate, pick through, and generally tear it all up. They will eat small amounts of the grass also, depending on their age. Usually, they really enjoy it; we never buy chick grit.
What is your mature chicken grit made out of? If it's oyster shell you shouldn't feed that to your new chicks. If it's some sort of rock like granite you could crush it up and give to the littles.