Whenever it is safe t for them to go outside on grass. Chicken feed and whatever they find in the yard should be the biggest part of their diet. They can have the occasional treat when they are around 6/ weeks old. You have chick grit for them so that is good for helping the gizzard to grind up...
Young chicks don’t need a lot of extras other than chicken feed and fresh plain water daily. Poops can change every time. If the chicks are active and eating well, there probably is no problem.