In regards to hikes that could be fun for a toddler, I would recommend not just hiking to Mirror Lake, but also doing the Mirror Lake loop trail. Also hiking in Yosemite Valley from the campground as far west (beyond Yosemite Lodge) on the Valley Loop trail should be rewarding. He could have some fun at the beach area near Swinging Bridge, or at the giant sand bar along Mirror Lake.
Other suggestions: hiking to Sentinel Dome and hiking up and down the Mariposa Grove of Giant Sequoias.
If they're still offering the tram tour this year at the Mariposa Grove (don't know when they're planning to phase it out), a less strenuous option would be to take the tram tour up to the top of the Mariposa Grove and then hike back down to the parking lot.
The Four Mile Trail shouldn't be too bad either if you schedule a break (like having a lunch break at Union Point) in the middle of the hike to ease the constant grind of an all downhill hike. (I would have him hold your or your wife's hand while hiking across the Four Mile Trail's traverse between Glacier Point and the first set of switchbacks. The rest of the FMT should be fine, though.)
Edited 1 time(s). Last edit at 04/10/2014 01:59PM by plawrence.