On the Title XIV laws, the only apply to "commercial purposes" defined as
``(2) Commercial purposes; engaged in the business.--
``(A) Commercial purposes.--A person shall be considered to
make a communication for commercial purposes only if such
person is engaged in the business of making such
communications.
``(B) Engaged in the business.--The term `engaged in the
business' means that the person who makes a communication, or
offers to make a communication, by means of the World Wide
Web, that includes any material that is harmful to minors,
devotes time, attention, or labor to such activities, as a
regular course of such person's trade or business, with the
objective of earning a profit as a result of such activities
(although it is not necessary that the person make a profit
or that the making or offering to make such communications be
the person's sole or principal business or source of income).
A person may be considered to be engaged in the business of
making, by means of the World Wide
Web, communications for commercial purposes that include
material that is harmful to minors, only if the person
knowingly causes the material that is harmful to minors to be
posted on the World Wide Web or knowingly solicits such
material to be posted on the World Wide Web.
Since an open source MMO is not exactly a commercial purpose, you probably don't need to worry about it. You would run into problems however if you started charging for access to the server (with intent of making a profit). If on the other hand you want to cover your @$$, it is fairly easy to use those printable forms that have to be signed by a parent or guardian, and then faxed or mailed in, there just has to be somebody who is willing to be in charge of receiving those.