I hiked the new Snoqualmie Falls trail with my 2-1/2 and 5 yr old sons back on Sept 21 2013.
The trail one-way is about 0.6 miles with elevation changes from 442 ft to 64 ft, according to my walking GPS. On that day, the lower boardwalk allowing views to the falls was closed. When I inquired, it was closed to install a new handrail. I suspect that has been resolved by now.
The base also has its own parking lot with excellent restroom facilities at N 47 32.731; w 121 50.437. This lot appears to be accessible via SE Fish Hatchery Road, albeit I didn’t confirm. Use maps.bing.com, enter “n 47 32.731; w 121 50.437” in the search box, press Search (enter) and then click on Bird’s eye.
The hike was short and enjoyable.