If they are smooth white kid, try Q-tips dampened with the suds from washing up liquid. Do it in front of the TV one night because you have to do it in small areas and it takes a lot of time. Don't soak the Q-tip because you don't want to make the leather wet, but you want enough of the suds to wash the dirt off. And don't use a harsh detergent - you want a mild dish soap - you can also use your own saliva as a test first - often that works even better than suds.