Remember not to treat this as a ‘who’s right’ exercise. There is no theory that is without merit. What you need to do is use the evidence of children’s language to question and weigh up the theories. Here’s a quick reminder:
Behaviourist Theory (e.g. Skinner) says that children’s learning of language is achieved by imitating adults, who reinforce their efforts either positively with praise or negatively with correction.
Nativist Theory (e.g. Chomsky) says that our acquisition of language is a natural, inborn process enabled by our Language Acquisition Device, which instinctively picks up the rules and patterns of the language around us.
Cognitive Theory (e.g. Piaget) tells us that children need to fully grasp the meaning behind words before they can use them.
Interactionist Theory (e.g. Bruner) recognises the LAD model but expands on it by stating that children also need a LASS (Language Acquisition Support System) to enable them to develop language.