When the king arrives, he brings the liaison with him
Back in version 0.28.181.40d, the outpost liaison became the Royal Advisor when the King migrated to your fortress. Since version 0.31.01, that doesn't happen anymore (the liaison becoming advisor, not the king migrating).
Also, in practice, they don't send caravans nor liaisons due to the [BABYSNATCHER] tag, that's not actually the reason they don't send caravans. They don't send them because by default, goblins are hostile to all other civs, and when they are hostile, they don't send caravans.
I'm pretty sure all babysnatching civilizations are permanently hostile toward non-babysnatching civilizations, so the only way you could get Goblins to be peaceful toward you would be to either make them non-babysnatchers or to make
dwarves babysnatchers.
Also, they can't send caravans at all; their entity lacks the tags to make them construct caravans, although they can still send pack animals.
You're confusing caravans with
wagons - caravans consist of both wagons
and pack animals, but wagons won't show up if the civilization is opposed to killing plants (which should only be the case for Elves) or if it doesn't have access to any wagon-pulling creatures (and Goblins can only use wagon-pullers if they are Evil or Subterranean, since they don't get access to any domestic creatures).