Finding a good long hike in San Juan County is not easy, and Moran State Park offers one of the best options. My family of four, with two boys ages 5 and 9, went over to the park for the day and enjoyed this amazing hike. Starting from the picnic shelter parking lot at Mountain Lake, you climb abruptly and relentlessly to the ridge that then traverses to the summit of Mt. Constitution. This climb is steep, no doubt, and views are few, but it feels great. Our two young boys had no trouble, but they are used to this sort of thing. It is a 1,500 foot climb from the lake to the summit.
Nearing the summit you get outstanding views out over the northern sound. Cypress Island, Lummi Island, and the northern Cascades all stand out. The stone tower at the top is always a highlight, though it is crude compared to the 13th century stone masterpieces my family recently saw on the island of Corsica off the coast of Italy. No cement there, just perfectly shaped and placed stones that have resisted wind and earthquakes for many hundreds of years. Anyway, the CCC work is still nice by 20th century American standards.
The drop down the back side of the mountain is very steep as well, and takes you down to Twin Lakes. The trail starts off the northern side of the parking loop near the tower. From Twin Lakes, it is an especially nice hike back along Mountain lake to your car.
This hike is completely clear of logs and in great shape overall. No mud problems and easy to follow. Though the altitude climb and drop may not work for all families and people, it is perhaps the most vigorous hike in the San Juan's and well suited to that purpose.
It is a 7 mile loop, and even our 5 year old had no trouble.