OK, I turned on my machine and re-ran the 'a11y/children' test.
It worked just fine under X and Xvfb when I ran it as just "./children" and "xvfb-run ./children" respectively -- I get OK messages like you wrote initially and no assertion failures.
Next I deleted the whole gtk+3.0 directory, redownloaded the source, and rebuilt it. I hit the same error:
WARNING **: Error retrieving accessibility bus address: org.freedesktop.DBus.Error.ServiceUnknown: The name org.a11y.Bus was not provided by any .service files
If anyone has an idea why running this before building gtk+3.0 is ok, but running it afterwards fails, I'd love to know. I can say that whatever the original problem (assertion failure in children.c) was, I didn't seem to run into it.