All of the words that the child has learnt have probably been learnt through imitation; the child would listen to his carers saying specific words and being rewarded for it. For example, saying specific lexis like "please" is something that the child has learnt and associated with getting what they want. Politeness is harder concept to understand at a young age, therefore proving that the child knows words like "please" and "ta" from imitation and reinforcement.
However, some words that the child has learnt would require more explanation and understanding that just simply imitation and reinforcement. This goes against Skinner's theory and shows that child language acquisition requires more depth than stated in this particular theory. For example, asking "wassat" requires a lot more knowledge and understanding as the child is asking a question, which is a concept more difficult to understand than simply imitating words that they have previously heard.