I know it’s illegal for a child to send nudes of themselves, as that would constitute child pornography. However, can a child ask a consenting adult for nudes legally?
There is nothing illegal about the child asking for nude images, but, depending on jurisdiction and circumstances, there is quite likely to be a serious legal problem with the adult providing such nudes.
The exact laws that might be violated would depend a lot on the jurisdiction. The detailed facts would also be significant. But such a situation is fraught with danger, for the adult. Even if there is no ill-intent, such actions could easily be misinterpreted, and might be technically unlawful regardless of intent.
Of course, not all nudes are obscene, or even "inappropriate for children". Which are so classified depends on the laws of the particular jurisdiction.
In germany, the child can not get indicted for a crime because children (defined by law as people under the age of 14) are never considered criminally guilty (§19 StGB). The idea of a minimum age of criminal responsibility is a concept which is common all around the world (except for Mauritius, Djibouti, Maldives and some of the United States).
However, the adult responding to such a request and sending pornography to a person under the age of 18 could be in violation of §184 StGB: Dissemination of Pornography:
Whoever, in respect of pornographic material (section 11 (3)),
- offers, supplies or makes it available to a person under 18 years of age,
incurs a penalty of imprisonment for a term not exceeding one year or a fine.
Now you might of course wonder what's about people who are criminally responsible (over 14) but not yet 18 and convince an adult person to send them pornography. This could be persecuted as Abetting (§26 StGB):
Whoever intentionally induces another to intentionally commit an unlawful act (abettor) incurs the same penalty as an offender.
Because it is typically illegal for an adult to provide a child with pornography, a child who requests pornography from an adult could face some kind of accomplice liability, on an aiding-and-abetting theory, a solicitation theory, or an incitement theory.
It's essentially the same as standing outside a store asking a adults to buy you alcohol.