There are numerous educational games designed to teach children about various financial practices. For example, there is the education-based economics simulator, Beat the Market Online or the World Game of Economics simulator that teaches about exchange rates and economic policies between countries.
These games do have value, but they often miss the opportunity to provide real, hands-on experience with the marketplace because they do not foster free trading between real players. More importantly, most students will recognize them immediately for what they are: educational tools. While some kids are willing to learn in this way, many balk at the notion of an educational game because it feels too much like homework.
The most valuable tool is actually more nuanced, and lies within games that children are already interested in playing. Sites like Neopets or Gaia Online attract millions of users of all ages, and popular multi-player online games like World of Warcraft have a massive draw for kids and adults alike.