Children of such a young age adapt extremely easily to the environment they live in. Case studies by Bickerton found examples of the above situation. Even though the sheer existence of UG puts the child's ability of learning a language to a great advantage, he or she needs to hear an existing language to learn it. Grammatical samples drive language acquisition. Plus , many people consider the grammar of working class folk to be poor, whereas, in fact, it is scientifically complex in differing ways, depending on the language.
Children do experience lots of positive evidence from their mother tongue language.
In many respects, this should make up for the impoverished negative evidence. However, the child still knows what not to say and what to say. This all miraculously happens without being formally instucted but growing up in normal environmental conditions hearing spoken utterances. The best part of these being produced by the child's parents. There is plenty of stimuli for correct, well formed grammatical sentences, but absolutely none, if any, for bad ones.