Vocabulary development in children is a real issue sometimes. Even for us as adults, we're able to comprehend much more vocabulary than we use in our daily lives. It's even truer for children.
One website I've come across is Freerice.com, and their mission is to fight hunger worldwide by donating 10 grains of rice for every right answer given on their site. It's neat too, in that children can see exactly how much rice they've been able to donate via their right answers. It's probably most appropriate for 3rd grades on up, but even adults will have fun with this site while doing something worthwile.