Children Avatars Are Allowed on mature sims if the person playing the avie is 18+ in real life age
i play a child avatar.
So Before you IM that child avatar or consider filing a report, for being in a Mature region or Adult - or for being on SL at all - read this:
"Child avatars are allowed within Second Life. A child avatar, even within a mature or adult region but not participating in adult activities, is allowed." http://wiki.secondlife.com/wiki/Child_avatar