Are you sure it's diarrhoea?
Every 4th chicken poop comes from a different part of the intestine (the ceca) and is soft, tan coloured and rather runny. That's normal.
Having said that, diarrhoea in chicks is very serious. It may be coccidiosis (a parasite picked up from the ground/droppings). If they look at all sick or are off their food then it would be sensible to treat as though they have coccidiosis (hunched posture, droopy wings, sleepy, ruffled feathers, sometimes blood in the droppings, pale faces/eyelids, and diarrhoea). The pet shop or feed store should have appropriate medication.
If they're happy, active and eating well, and don't have the 'sick bird' posture or pale faces then you might try some yoghurt with grated carrot and rolled oats as a probiotic treat on top of their usual food, and not worry too much about the odd runny dropping. But take any sign of going off food or having droopy wings or hunched posture seriously.