You speak of "using Insight" like "using a hit die." Without sufficient interaction to actually determine something is amiss with the "little girl," attempts to discern her true intention or nature is a fail, no roll, in my view. It takes time to figure this stuff out. "There's a cloud of poisonous mist rolling down the slimy slope and an ominous croaking coming from somewhere behind it. The little girl coughs and cries 'My eyes are burning!' as she tries to cover her face in [wizard's] robes. What do you do?"
I fully expect the players to be suspicious, so I'm going to put that little girl right up in their grills and see what they do about her in the face of other dangers. That's an interesting scene in my view, one that could play on personality traits, ideals, bonds, or flaws.
I grab the little girl and throw her out the portal behind us. Better on the mountainside than in an evil, world destroying demi-plane with a poisonous cloud rolling down on her. This would mean that there would be one slaad in front and one behind when it ran back in, but if it ran back in at least we know it's not a normal little girl at that point.
