Hi Will. NIce translation!
Let me ask you:
First stanza - Since they are talking about the fact that everybody takes part in life: big people, small people, the poor, the rich...etc., is it all right to say "play" and not "plays"? Are you making a single subject out of "the big one, the small one"? Because, in Spanish, the verb repeats and it makes it two different ones.
2. Third stanza - "Play with your clean letters". They are comparing with a game of CARDS.
Best to you. Thank you
Juega con tus cartas limpias (Play with your clean cards) = Means to play honestly or to play without cheating; Play cleanly.