Well, (donning web administrator hat for a moment) if by "kid" you mean a child under the age of thirteen, it is PROBABLY forbidden in the first place by the web-host.
There are special rules about what constitutes an "age of consent" under US federal law, anyway. This is the underlying reason why most public websites specifically prohibit use by anyone under the age of 13. Kids that young CANNOT use the web without specific parental supervision and approval.
My daughter is highly internet savvy, and I will say that this has been an on-going problem for her and her friends.
That is, they love having websites devoted to special interests, but the problem is that it is all technically illegal for many of them because of their ages, and you're right-- anything "free" is also loaded with advertising and probably spyware/spam as well.
One tactic that they've used is to manage a message board which has fairly tight privacy settings. That way, (as in some Yahoo groups) the conversations are completely private (well, as private as anything is in cyberspace) and only INVITED members can join, by asking the site Administrator for permission.
Other cautions for kids on-line:
Never share "personally identifiable" information.
This is a large and nebulous definition. With kids that have trouble with non-literal interpretations, this is a big stumbling block. Location, for example, is something which can be discovered a variety of ways:
a) FB style "updates" in real time can lead someone to easily determine your time zone, and from there, small snippets of daily information (about weather conditions, school schedules, etc) can provide more and more details about absolute location.
b) IP addressing. Just because YOU don't see it appear associated with a visit to a website? Don't believe for an INSTANT that it is private to everyone else. At the very least, the website administrator knows where you are via IP tracking.
Both provide some great starting points for developing a family AUP (Acceptable Use Policy) or "contract" that your child(ren) will have to abide by in internet use in order to retain the privileges they want.
The Pew Research Center has a few useful things, as well (and some of those discuss federal law):
And just in case you were thinking that the FTC isn't serious about enforcement:
Wikipedia entry for COPPA. (Check out the list of companies that have been charged with violations! Wow.)
The important thing to note about COPPA is that most third-party hosting sites must ALSO obey it, which means that 'free' webhosting services will almost certainly not allow content intended for users under 13. (The site that I help to run has this problem, actually-- because we are hosted for free, we have to abide by our web-host's ToS.) Collecting IP addressing information or cookies either one may be considered "personally identifying" information.
Here this thread is. I got this email about smart phone gps embedded in every picture taken on your phone. I just think it's strange, not really as scary so much. I might feel different if my baby's were teenagers talking online.