Halliday (2008: 179-80):
In terms of children’s early language development, the interpersonal metafunction provides the prototype of how meanings come to be grammaticalised. The two systems that were first grammaticalised by one small child (Nigel, aged 0;10) were:polarity: positive / negative modality: value: low / high
followed shortly by the two forms of “appreciation” in conjunction with the feature “positive”:appreciation (positive): impact (“that’s interesting”) / quality (“that tastes nice”)
Note that these were not yet mother tongue; they were protolanguage, realised by sounds and gestures. But they were systemic, or at least proto-systemic; and they provided the model for the linguistic systems of appraisal, where each lexical item realises the intersection of an appraisal feature with polarity and/or modality.