Patterns of errors, e.g. substitutions of a particular sound for another (or one group for another like a preference for plosives over fricatives) - remember to look for consonant blend reductions, deletions, additions, reduplications etc.
Think about the ‘fis’ phenomenon and what that suggests about children’s acquisition of the sounds of language.
Think also about children’s pre-verbal development and their response to sounds from birth.
Written Language Acquisition
Although phonology does not seem relevant to written data, remember that children’s: early writing is strongly affected by the sounds of words, as children learn phoneme-grapheme correspondence.
It’s worth commenting on:
Choices of graphemes for different phonemes (e.g. choosing “ee” for “y” in “dadee”)