you can talk about bruners social interaction since that's the key theory. this question seems to be mainly centred around the nurture debate so any theory you can bring from the previous question above. you'd obviously need a counter argument with reflects the 'learning vocabulary and grammar' bit, obviously adhering to the innateness debate. since i don't have the transcript, i can't make precise notes but you can refer to how a child acquires morphemes and negation naturally (brown and bellugi), you could also talk about certain halliday functions since it references pragmatics. hallidays functions aren't always relating to innateness so be careful which one you choose. you can also talk about skinners behaviourism but only with social interactions since he talks about positive and negative reinforcement.
you can also use phonological adjustments (idk what they're called) but subsitution deletion and so on, they cover how kids may reject certain syllables no matter if encouraged.
i hope this was of some help, i kinda just dumped a lot onto here :,)