Now don't go jumpin' on me, but I started ( and mind you this was a long time ago, ) making it really sweet. Gradually I reduced the sugar to a comfortable level. I send my kids yogurt (b/c I don;t have kefir grains anymore) to school with just a bit of jam mixed in.
for breakfasts sometimes, we make smoothies with frozen blueberries, raspberries and bananas, with a cup of yogurt. The bananas are used mainly for sweetening, although the flavour is nice too. Peaches and pineapple are good for sweetening naturally also.
My kids love smoothies, but they have to have blueberries in them or they will pass. find out what your kids like. If they are not big on fruit, try chocolate peanut butter or chocolate mint-wit cocoa powder and extract, (and sugar)..Experiment.