There is a couple ways. The first is the Christmas gift fund, which is divided between all the kids in the program to make sure even unsponsored childred get a Christmas present. If you want it to go directly to your child it's best to send a Child Gift. I believe you can do this online or by phoning/emailing. Note: It takes time, so if you are just sending it now, the child will not receive it probably until around March. That being said, they will still appreciate it very much.
One more thing to note is that your child WILL have received a gift for Christmas already through the Christmas gift fund whether you sent money for it or not. (They might even write to you thanking you for this gift