Little Lakes Valley is the major draw for most folks, so get to Mosquito Flats trailhead parking early as it fills up quickly. My daughters started hiking on their own at 4 or 5 years of age (mostly). If not, they would get a free ride on dad’s strong shoulders.
The good news is the lakes with fish are not that far! The first lake out of Mosquito Flats is Mack Lake at 0.5 mile, Marsh Lake is about 1.0 mile and family favorite Heart Lake is about 1.20 miles. There is one hill that’s called “Crank Case Grade” where it may slow down your boys a little. You’ll catch mostly Brook Trout in these waters, but Rainbow and some Brown Trout are present too.
Of course you can just drive to Rock Creek Lake and fish along the shoreline just about anywhere for larger trout if the hike up Little Lakes Valley doesn’t pan out. The possibilities are endless. Have fun!