As everyone else has said, you do not need to give them grit if all they eat is the chick starter. I personally like to give them grit on about their third day since by then they should have figured out what their main food source is. They may not need it, but their digestive system is set up to work a certain way. Why not go along with that if you can. Besides, I like for them to have grit to help their system if they eat anything other than the chick starter, for instance if a hard shelled bug wanders into their area or maybe they eat some of the wood shavings or whatever you use as bedding. It would be nice if that could pass through their system. Many people do not give their chicks grit and they do fine. It is your personal choice.
Be careful of the grit from the pet shop. Some of the parakeet grit contains extra calcium which is bad for a growing chick. Read the ingredients label carefully.
For chick grit, I sometimes use construction sand, the coarse stuff, not the fine smooth play sand. I have also been known to go to a gravel road or driveway and collect some sand and small pebbles. If you roads are salted for ice this is not a good idea since they cannot handle the extra salt well either.
Good luck!