Here's the definition of obscenity under the Miller test (1973):
1.) Whether "the average person, applying contemporary community standards", would find that the work, taken as a whole, appeals to the prurient interest,>In its 6-3 decision written by William J. Brennan, Jr., the court held that material being obscene depended upon "whether to the average person, applying contemporary community standards, the dominant theme of the material taken as a whole appeals to prurient interest." With its emphasis on the reaction of an average person rather than that of an especially susceptible person, the court rejected applying the Hicklin test as a means of determining whether material is obscene, and the ruling represented a liberalization of the nation's obscenity laws.>prurient simply means arousing or appealing to sexual desire.
2.) Whether the work depicts or describes, in a patently offensive way, sexual conduct or excretory functions specifically defined by applicable state law,>The phrase "patently offensive" first appeared in Roth v. United States, referring to any obscene acts or materials that are considered to be openly, plainly, or clearly visible as offensive to the viewing public.
3.) Whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.
The work is considered obscene only if all three conditions are satisfied.
So in other words, regular sex, nudity, orgasm and stuff that your average normal person would find attractive is off-limits, the government won't do shit about it because it's protected speech. But fetish shit is definitely obscene. If you were to draw children engaging in regular sex acts like performing coitus or whatever, that should be fine, but if they're shitting into each other's mouths or engaging in extreme, violent sadomasochism, then yes, that is still illegal, and the fact that it's drawn/computer generated/written doesn't change that fact.