Perhaps the most important source for data on language acquisition by children learning a variety of languages is the Child Language Data Exchange System (Childes). This includes audio and video recordings, as well as transcriptions, of the speech of children.
The classic article Berko (1958) (PDF, 284 KiB; referred to in Question 4, p. 222) and the full set of wug pictures (ZIP, 5.3 MiB) is available at Childes.
