Check the calcium content of that parakeet grit. Some has added calcium. Excess calcium can harm a growing chick. You can probably get chick grit at the feed store or just give them coarse sand and small pebbles. They naturally pick up grit from the ground as long as it has small pebbles in it and most ground does.
I agree. As long as you only feed them the prepared chick feed or a few other things, you do not need grit. Pellets and crumbles have already been ground down and then reformed so it will break down in their system without grit.
When a broody hen takes her chicks off the nest, usually even before she takes them to water she has them pecking at the ground, picking up grit and other stuff for nutrients. Some of that stuff they pick up needs to be ground in their gizzard. They get grit from the ground. It doesn’t take her long to have them eating grass, weeds, and other stuff that needs to be ground up. She’s feeding them “treats” from the very beginning. In her case those treats are varied enough that they are a basic part of their diet. In our case, since our treats are not nearly as varied, the bulk of what they eat should be their basic feed, with treats offered like you would offer candy to a kid. Some doesn’t hurt a bit and makes life more enjoyable, but they don’t need a steady diet of it.
In my opinion, the best thing you can do to raise healthy chicks is to keep their environment as dry as you reasonably can. Once they go outside and the weather sets in wet, the ground will be wet, nothing you can do about that. But keep the brooder and coop dry. That’s important.
My second suggestion is to not keep them in a sterile environment. Strengthen their immune system by exposing them to dirt. Don’t let the poop get out of hand, but don’t try to keep their area spotless. Poop management is important, but don’t obsess over getting out every speck. One way the exchange probiotics and build flock immunities is by eating a bit of each other’s poop. But for sure, don’t let the poop get wet. That’s unhealthy and it will quickly stink.
Welcome to the adventure. You are going to have fun.