Final answer:
A child correctly using irregular past tenses of verbs is in Roger Brown's Stage II, which showcases growing complexity in their sentence formation and use of inflections.
Step-by-step explanation:
A child who correctly uses irregular past tenses of verbs would be in Roger Brown's Stage II of Syntactic and Morphological Development. This stage is characterized by the use of simple sentences and an increase in the complexity of speech, including the acquisition of inflections such as the past tense. Children at this stage often begin to correctly use irregular past tense verbs, such as 'went' or 'saw', which indicates their advancement beyond the simpler, more regular forms that are prevalent in Stage I.