There are at least three key words shared by the two sonnets, argument, invention and verse, and the content and placing of the two cannot be accidental. But there is no explanation available for the significance of the number 38. Shakespeare was 38 in 1602, and there may be some relevance in that fact, especially as age, decay, baldness, loss of beauty, decrepitude and death feature so much in the sonnets. It remains an open question when the sonnets were written, or what span of time they cover, either in their composition or in living experience. Certainly it would be fascinating if we could solve that puzzle, but no information that we have at present is likely to be sufficient to resolve the questions for us.
Poets often refer to their verses as their children.
The metaphor is one of ornamentation in general, especially of clothing - the verse is bare of new styles of literary decoration. The clothing metaphor is further developed in line 6. By new unfolding his imprison'd pride. But, rising at thy name, doth point out thee As his triumphant prize. Both variation and quick-change seem to be musical terms, but OED gives the former in its musical sense as dating from only 1801.