Welcome to the forum, glad you joined.
This is an area (like practically everything else on here) where we can all have different opinions and a lot of personal preference. I'll give you my opinions and what I do.
My broody hens start giving their chicks what many would call treats shortly after she brings them off the nest. The ones that hatch in the coop usually eat chick food and drink water for the first couple of days because that's all they have until she takes them outside. The ones that hide a nest and hatch outside the coop start them immediately because they have better access.
I personally like to wait a couple of days after they learn to eat their chick food before I start on any treats for my brooder-raised chicks. Some people start treats immediately like the broody hens. Just remember that the chick feed should be the vast part of their diet. The chick feed is a complete balanced diet and contains everything they need for growth. Too many treats can upset that balanced diet, even if it is a very healthy treat. Too much of a good thing is not a good thing. To grow up healthy and strong they do not need treats, their chick feed by itself is sufficient.
They do not need grit if all they eat is things that do not need to be ground up in their gizzard. Chick feed has already been ground up, no grit required. There area few treats that do not require grinding, like boiled eggs. But it's a good idea to give them grit for most treats just in case. Anything in chunks needs to be ground up. Green leafy vegetables or even grass needs to be ground. A broody hen takes her chicks to a bare spot of ground right at the start so they can peck at the ground so they can find grit. I feed my brooder-raised chicks dirt from the run on their second day in the brooder. Not only does this give them grit but it strengthens their immune system and gets the started on the flock immunities they will need and gets probiotics from the adults in their systems. If you don't have adults you won't get all these benefits but I think it is still a good practice. If you don't want to feed them dirt you can probably get chick-sized grit at the feed store. Depending in what you feed them they don't absolutely have to have grit and many people raise their chicks never giving them grit, but I think it is a good practice.
What can you give them as treats? Practically anything you can give an adult chicken. That's what a broody hen would give them. You may need to break it up into bite-sized pieces, broodies do that for their chicks with bigger stuff. Just don't feed any of it to excess. Rely mainly on the chick feed for their balanced diet.
Other than occasional dirt from the run I don't give my brooder-raised chicks any treats until they are out foraging with the adults, usually around five weeks of age. Then they get whatever they can find for themselves. I often toss garden and orchard waste and excess and kitchen wastes to them all, adults and chicks.
