Children learn functional units of language as a short cut to learning their language. Specifically, word endings and allowable combinations are learned before being mixed and matched to previously learned words and word endings. So, in order to understand or produce a word, it isn’t necessary to have already used that word. The child merely has to learn the parts and arrangements. Consider these words:
walk, walked, walking, walks
talk, talked, talking, talks
To be able to comprehend and use these eight words, only five things must be learned. 1) walk; 2) talk; 3) -ed ending; 4) -ing ending 5) -s ending.
This fully explains why children produce constructions like “goed.” The child doing this has learned 1) go and 2) -ed ending. This child has not yet learned specific exceptions to how words should be combined.