How do we know that the girls are sitting in individual containers, and not in one container for all of them? The English sentence is ambiguous about whether it is one or many.
Since the English sentence is ambiguous, both "miben" and "mikben" should be acceptable.
Could this question be asking about what the girls are wearing while sitting?
"Miben ülnek..." could have that meaning, albeit it's not likely to actually be used like this. "Milyen ruhában ülnek..." is a bit better, but still not the most natural.