That "and" is worth a great deal of attention. I would write "Summertime when" but that "and" sets up a tone, a whole poetic tone, not to mention a whole kind of diction that is going to be used in the play; an informal, uneducated diction and a stream of consciousness, as in many of the songs like "My Man's Gone Now". It's the exact right word, and that word is worth its weight in gold. "Summertime when the livin' is easy" is a boring line compared to "Summertime and". The choices of "ands" [and] "buts" become almost traumatic as you are writing a lyric – or should, anyway – because each one weighs so much.