Hmmm, If you take a stroller, you could have a heck of a lot of fun walking the Malecon in Mazatlan and the entire downtown area (cathedral, mercado, etc.) as well.
Ditto for Puerto Vallarta. Great shopping, good restaurants and lots to see. If you want to spend the day at a child friendly beach club, consider Los Veneros on Punta Mita. It comes complete with a water park for tiny ones. Great beach, excellent restaurant, bar, amenities and so on. Your entire family would love it there as would your 3 year old.
Just some thoughts.