If an MMORPG had "100 year old elvish" NPCs which had childlike features and full nudity, then playing the game might be distasteful but probably should fall under constitutional protection.
The law is very cognizant of intent and willfulness. If you willfully intend to break the law, but fail to actually break the law, you can still be charged and convicted of a crime. I don't have a problem with that.
I think you must agree that if you solicit a bot then there is no crime, just like if you "murder" a bot there is no crime. Ultimately we have juries to decide of the accused if full of shit claiming they were soliciting bots when they were actually soliciting children. The burden of proof falls on the Government to prove this beyond a reasonable doubt.
As bots get increasingly realistic and entertaining enough to create a market for that service (regardless of the physical characteristic of the avatar) it will be interesting to watch jurisprudence evolve.
A convincing avatar which can be synchronized with a human's text input, and/or speech alteration (not text-to-speech) is one way you approach 'the singularity' without having to synthesize human thoughts and emotions. Such a system, if it can cross the uncanny valley, would radically improve quality of live for millions of (and I hate this term) 'genetic lottery losers'.