I don't know where you are in the world and I don't know what rules apply there, so I can't really help with the wording.
Possibly make a difference between real human nudity and doll nudity? A doll is a created object, after all, not a person with feelings and a right to privacy.
When it comes to the over all philosophy around it, though, it could be argued that doll making is sort of a hobby for adults anyway. I don't mean 'adult' as in some arbitrary number of years old, but more in the sense we tend to use tools and materials that require a certain level of maturity to use responsibly.
And when it comes to the BJD hobby in general, the same is still true. To some extent, at least.
These dolls are not play line dolls made for children. In fact most makers will clearly state that their dolls are not intended as children's toys, but are exclusively intended for an adult audience.
On the other hand, I don't believe for a minute that anyone could possibly be shocked seeing naked doll parts, after signing up for a doll making forum, regardless of age. But I know some places have laws that are kind on puritan and not very well adapted to reality, so, well... You do what you need to do.