when i used to make a lot of yoghurts and my boys were young and didn't like 'bits' in them, i used fruit flavourings that you can buy from the kind of shop that sells cake tins and icing sets, etc. or cocoa powder or hot chocolate. you can get jars or bottles of fruit puree stuff for pouring over ice cream, which work well stirred through yoghurt.
to be really healthy, what about fresh fruit, or tinned? pie fillings are another option too. or honey.