Used largely as part of the Nannie Peak Loop, Walupt Lake and Creek are a nice little hike in their own right. While not big on views, the flat grade of this trail makes it a good outing for families, since the river partway through the route offers a good turnaround point if little legs get tired. If you want to keep exploring, head up to where this trail terminates at the Pacific Crest Trail, and see if you can meet some thru-hikers.
Starting in the Walupt Lake campground, set out on this flat trail. Less than 100 yards down the route, you'll need to fill out a Goat Rocks Wilderness permit (they're free, and kids might be interested in helping the group out by completing it).
Continue on down the trail, enjoying the cool forest and peek-a-boo views of the lake, as the way undulates gently along the hillside above. There are several short but steep trails that provide access to the lakeshore, though the beach back at the campground is probably a safer option for little ones.
Step carefully over rutted, rocky gullies, leftovers from when winter snowmelt swells streams. While there may not be water running in them when you visit, the rocks may be loose, so watch your step!
All around you there are second-growth trees and vivid green groundcover, including mosses and even vine maple in some sections. Before you know it, you'll make a somewhat steep climb to a high point. Listen closely here; the river you're about to come to is within earshot. Descend and hike upvalley just a bit, coming out on the banks of slow-moving Walupt Creek. There are two logs set up to help you cross with dry feet, but be aware that in high water (early to late spring) this may be a dangerous ford.
Stop and rest here, or cross if it's safe and your group has the energy. Continuing up, follow a rooty, rutted path out of the river bottom and up to where the tread improves. The trail here climbs more steeply, and in a mile and a half you're carried up out of the dark forest into an airier, light-filled, fragrant woodland. Small camps and water sources to the side of the trail indicate usage from PCT thru-hikers. These folks pass through from mid-July to late August, and you'll likely meet some at the junction 4.4 miles from the trailhead, where your hike ends.
WTA Pro Tip: To continue on, it's possible to loop this trail with Nannie Ridge-Sheep Lake to make the Nannie Peak Loop, or to head south on the PCT through a marshy bottomland, connecting with the Coleman Weedpatch and then walking 1.5 miles back to the campground from that trailhead.