My chicks had diarrhea when I first gave them treats, so it's probably the change in diet.
Chick grit is a must! Dirt could work, but it would be better to get a bag of chick grit at your local feed store until they can get it for themselves outside. Walmart might even have some kind of grit for parakeets, etc.
Bananas are ok for chicks to eat just not too much.
I know it's fun too give them treats, but I would wait until they are atleast a month old.
I just started giving my 8 week old treats a couple times a week. It's fun to watch them play keep away.