“Let’s get to work” is a common way of saying, “OK, kids, stop playing now, there’s serious business to be done, no more fun.”
But when we’re having fun, we love what we’re doing so much more than when it’s drudgery.
“Let’s get to play” sounds much more inviting. For the musician or the athlete, to play is to work. So it is for the rest of us, when we’re in the right place.