Sadly it's not that simple. Even if that were the only rule, illegal where? The rules aren't even uniform among the 50 US states, not to speak of city/county level, or other countries.
I think that the (unique?) problem with child issues is that if you allow things that are clearly not illegal (just 'creepy'), you'll draw a community that 1) Looks illegal from the outside and 2) Will, with very high probability, be doing illegal things over the messaging system or private/side boards.
To the greater world, 1 and 2 are enough for a high profile lawsuit or media blitz that would effectively be a death penalty for the community.
The problem is that criminality is decided by a jury if a case like this goes to trial. In New York or San Francisco or even D.C., getting an obscenity or child porn conviction based on images of minors that are not nude, whether they're fully clothed or wearing swimsuits or bras, would be difficult. Getting an obscenity conviction for fictional depictions would be nearly impossible in those locations.
Then there are cases like the Christopher Handley manga case in Iowa, where he plead guilty to avoid the risk of a longer sentence for possession of purely fictional manga that a jury of his peers may well have deemed "obscene".
