Fat Man's Pass is a fun little geological oddity. It's a tight squeeze between two large boulders. The passage is short, but getting through it is difficult for some. Other boulders and little caves make this a fun spot for kids to scramble around and explore.
The hike along National Trail is pretty rewarding itself. It passes through a healthy saguaro forest growing among the many rocky outcrops atop South Mountain. It is nearly 2 miles from Buena Vista Trailhead to the junction of Hidden Valley Trail, where Fat Man's Pass is located, but the way is fairly easy. It follows rolling terrain with only a few rock stairs as obstacles. The pass isn't immediately visible from the main trail, but it is not hard to find. When you reach the signed junction with Hidden Valley, turn right and go down a little hill to find the boulders.
The higher elevation generally makes this area breezier and more pleasant than down in the valley, but temperatures can still exceed 100 degrees. Avoid hiking during the hottest part of the day and bring plenty of water. Also, always be sure to stay on the trail or on hard rock surfaces to avoid damaging the fragile cactus forest environment.
Note: Mountain bikers and horseback riders also use National Trail frequently, so take care and watch for other users. Dogs are allowed on leashes.