The complexity of children’s utterances (esp. morphologically) - remember to comment on what the child CAN do, not just focus on errors.
Virtuous errors - and better marks are likely if you explain them as overgeneralisations of particular rules.
How children form negatives and questions.
Grammatical ‘virtuous’ errors can be used as evidence in support of the nativist theory, suggesting an instinctive absorption of rules and patterns. Such errors can also be used as evidence to question behaviourist assumptions.
Written Language Acquisition
It’s worth looking out for:
Sentence types and how clauses are connected.
Use of particular word classes, e.g. adjectives/adverbs to add detail.