I think both Jack and Tai's approaches have merit in different situations. With people you know, are familiar with, and interact with constantly - such as a guild - it is best if the guild knows your child's age so they can act more age appropriate when the minor is online. For example, I was once in a guild which was very good at moderating swearing when the guild minor was online.

But Jack has a very VERY good point too. There are alot of creepy people on WoW. And they're creepy even for GROWN ADULT women. I wouldn't want my girls within 10 leagues of said creepy people, and sometimes they get creepier when they know the person is a child. Some people are just jerks and like to take advantage of naieve players, and lets face it, there are many naieve players which are minors. (Of COURSE there are plenty of mature minors who are not naieve, too) I support the "Don't ask, don't tell, EVER" policy in all situations outside of a guild you trust. And frankly, if you have a creepy person in your guild (been there), extend the policy to the guild. Let the GM know about the minors so they're not in the dark, and hey also let them know why you've told your kids not to tell their ages. Most GM's would understand I think.