Toontown can certainly be addictive. I should know, as an adult who plays way too much. I think it is important for parents to realize there are many adults who play. Most of the adult players I have met started playing with their children, and then discovered, as I did, that the game contains sufficient depth and strategy to be interesting for players of all ages. Fortunately, the adult players I have met appear to have the best interests of the children at heart, and work to keep this a child friendly game. With that said, there is always the potential for a child to be approached by an adult with criminal intent. For this reason, it might be a good idea to not permit your child to have open chat (they can be limited to the speed chat). However, open chat is almost necessary when/if they reach the higher levels of the game, as many players will not invite chatless toons into battle groups. I eventually "caved" and gave my daughter chat, with clear instructions (I hope) on safe chat vs. unsafe.
Other comments -- she learned a lot of math early on (number of points to defeat a cog), and her typing skills improved once she got chat.
I was initially taken aback by the cartoon violence, and somewhat concerned about its psychological effects. But ... the enemy is always a robot. Does it really harm a child to see a piano dropped on a robot? You have to decide that for yourself. I rated this for 7+ because of the violence and chat issues, but my daughter started much younger.
I like the cooperative aspect of the game. Occasionally there are toons or groups (bored teens?) that will attempt to sabotage other players, but almost all toons, kids and adults, are in the game to have fun and work together.
All in all, it's a fun game, but you may want to watch that it doesn't take over your child's (or your own, lol) life!