When reviewing a game for its suitability with children, I use a fairly broad rating system. I think utilizing anything with a numerical scale, decimal points, bell curves, or other fancy equations isn't really beneficial for the purpose of simply deciding if a game is right for your kids. That's mainly because these are all based on playing with my own children, and your situation may vary.
A for Adults I believe adults will enjoy these games a lot more when played with other adults. Typically these games will involve deep strategy, a complex rules set, or mature content. Since I mostly game with my kids, I probably won't review many games with this rating. But just in case...
T for Teens I believe these games are well suited for junior and senior high kids. The rules tend to be moderately complex and/or the game requires a bit more developed and nuanced strategy. Generally teens will need more than one play to explore all mechanics and intracacies. At the same time, adults will enjoy these games with their teens or at peer game nights.
K for Kids I believe these games are aimed more predominantly at younger children, say between ages 4-12. Parents will enjoy playing these with their kids, but primarily for the fact of engaging in quality time with them. These games will find limited, if any, game time in adult groups.
E for Everyone Fun for the whole family! I believe these games provide a balance in ease of play and meaningful strategy for all ages. With smooth mechanics that children can grasp and explore, they still have enough depth for kids to gain experience and for adults to master against peers at a group's game night.