دَخَلَ الْمُعَلِّمُ وَالْأَطْفَالُ جَالِسُونَ.
The teacher entered while the children were seated.
On this reading, the nominal clause 'the children were seated' describes a state concurrent with entry. Compare:
دَخَلَ الْمُعَلِّمُ وَجَلَسَ الْأَطْفَالُ.
The teacher entered, and the children sat down.
The latter may report two distinct events in sequence. The contrast is not merely that one clause is nominal and the other verbal; it is the difference between a simultaneous condition and another occurrence. Context remains decisive.