2 kids under 4 (and another due next month) fixed my back pain. Turns out that constantly picking up babies and toddlers is the exact amount of exercise my back needed.
Be careful, I'm currently in treatment for a shoulder that has given me significant trouble and I suspect it was due to picking up my (bigger and heavier than average) son. Not that I have hard evidence, but it was pretty much the only frequent physical activity that could strain my shoulders when the problems started, as I didn't exercise apart from long walks.