Well some Kids Websites to my Knowledge do have Forums. An example of this is the Kids Game, ROBLOX. All though ROBLOX doesn’t have its Forums anymore, it still was a Kids Game and had Forums. It also did censor a lot of Words for people under 13.
It is banned for a reason and I believe requires parents or guardians to mail in permission slips under the law.
The reason for that is that paedophiles tend to gather around places with a lot of children.
It's not the classical argument of "are the children mature?", it is literally that you can and will get way in over your head in trouble.
It really isn't worth the hassle and it's legally dubious to begin with.