We get a season pass here and we already have access to the private beach on the East end, I think that speaks volumes to how much we like this beach. It has two playgrounds perfect for kids of all ages (but no baby swings). The beach is long and wide, so you can always find a spot. There's sun AND shade. Great swimming areas and great fishing areas. They also have rentals for all types of fun activities - I recommend trying the paddle boarding and the paddle boats. They also have a snack shack so you don't even have to pack a lunch. But if you do want to bring your own food, there are plenty of picnic tables and bbq's. This is my personal favorite family spot in Truckee.
Families who want a quiet, old-fashioned vacation in the mountains love Donner. On the 7.5 mile shoreline, you can camp, launch a boat, rent a cabin, fish, hike, ski, and enjoy the crystal-clear, blue waters. There are nearly thirty public piers on the north side for fishing and boating. Shoreline Park offers bank fishing and a pier, boat launching, picnic sites, and swimming. On the west end, a swimming area is supevised by lifeguards.
Above the lake on Highway 80, the Emigrant Gap viewpoint on Donner Summit, at 7,135 feet, is a must stop. Your family will get a dramatic geology lesson when you look out over hundreds of miles of high country to see the tremendous tilted block of the Sierras sloping toward the west. Glacial canyons are gouged out of the granite, and the Yuba and Bear Rivers have cut their own valleys.