Being nice to a child may not seem like a hard thing. But what if it's your own child? Are you nice all the time? What if you don't have children?
The holidays are a magical time for children. They'll never be little again and its a chance for us adults to show them the magic of the season.
If you have children of your own - start a holiday tradition. Whatever it is - do it every year so your children can have something to hold onto and look back on as an adult. Maybe its eat Chinese on Christmas Eve or go to the local Christmas Parade or bring presents to a homeless shelter. Find something that makes sense and start your own tradition.
If you don't have children, maybe you have nieces or nephews or maybe your only encounter with children is those you see in public... smile at them. Wave at them. Say "Merry Christmas". Help them feel the love of the season through everyone around them. We don't want children to see how mean adults can be - especially at Christmas time.