To stay healthy you should consume enough water in a 24 hour period that you pass a minimum of 128 ounces or 1 gallon through the bladder. That, or about 3 liters to keep your organs and joints...
No, they will get diarrhea at any age. Get some "plain" yogurt, no flavoring, there are a few brands, Mountain something is good. You can add a little water to it, my teacup loves it and she is 5...