If your chicks are on starter mix, the bag will tell you when to change to layer mix (assuming you have hens!). I think it is at about 17 weeks, if memory serves. You shouldn't need to supplement calcium--there is enough of everything they need in the starter mix.
If you want to give them treats make sure you give them small bits of fruit like grapes, or vegetables like squash. Don't give them anything too hard, or long which can lead to crop problems.
Just keep their area clean, make sure they have plenty of fresh water, and feed them an approved starter mix (I use the brand with the checkerboard). You are doing fine, and if you need a true expert's advice, ask Velvet Sparrow at her website. She is the authority on poultry!