There is an Adoption.SE under consideration in Area 51 at the moment. One discussion post states that it is already covered by Parenting.SE:
Adoption is certainly a very worth-while topic (speaking as an adoptive parent myself) - but it is clearly covered by Parenting.SE.
As one response to the discussion question puts it:
Adoption doesn't just affect the people adopting, it also affects the people being adopted, the people giving up their child(ren) for adoption, and the birth/adoptive family of the adoptee. There are plenty of adoption related issues that would be off topic at Parenting SE....
The only discussion of adoption-related issues on our meta is Are questions about becoming a parent on topic? which concluded yes, they're on topic. (In addition, we've got a handful of questions tagged adoption.) There isn't much discussion beyond that, however. It's still early in the beta process, so not clear whether Adoption.SE will be able to take on questions that we may be unable to answer. However, it may be useful for us to better define adoption-related questions. We've established that "adoption" is on topic for our site, but are there related or sub-topics that might not be on topic?
To be clear, I am not asking this question to detract from that site's validity or potential in any way. If they successfully launch, then we know what sorts of questions would be good candidates for migration (e.g., questions we can't answer). If they don't successfully launch, then we can still provide a StackExchange location for adoption-related questions (although perhaps with a narrower scope). Either way, having a meta discussion can better define topicality of a range of adoption questions, useful to refer to in future.