I have had some experience directing children in the past. Granted, the children I was working with weren't actors... they were students at a local elementary school participating in a set of skits for the district's Bus Safety Video.
However, I did learn many things that would be helpful to anyone working with younger individuals in any capacity. Really, there's only one major point to make about working with younger actors...
#1 - Patience
This will be the key factor in the working relationship. The more patience you show for them, the more likely they are to pay attention and work with you.
As far as anything else goes... this will be true throughout. Also, it's good to realize that they are children. When there is down time keep them busy. Have them work on their material, though, unless they're professional child actors I wouldn't suggest overdoing this... Also, child actors should realize how much fun they're having... so over all, make sure that what they're doing is fun.
Maybe you can make the process of working over their material more fun... make it into a game, something subtle that they wouldn't even notice. Also, be sure to break more often for the children than the adult actors. All work and no play, just like jack, makes children dull.
Anyhow...
Go forth, create, and above all... Have FUN!