We backpacked in the five miles with my wife and five kids and had a great time. The elevation change is deceptive on the topo maps the lake is only 600 feet above your start point but careful observation will note the 1800 feet up and 1200 feet decent back to the lake. Beyond Peterson Lake are two more lakes about three miles further and worth the hike as well.
As noted by others, if you are looking at a day hike this would be a tough hump for kids.
Gary D. on Sweeney Ridge Trail to Peterson Lak...
Hiked this trail last month with my son. Terrific scenery from a rock overlook. Wild flowers & Bear grass were in bloom. Bypassed Peterson Lake after a couple & fishermen said they were run off by flies & mosquitos. They were correct. Continued to Lake Duffy & took a quick cold dip. Take plenty of water or be prepared to refill from springs.
This trail is also called Sweeney Ridge and it kicked my butt. The FS road to reach the trailhead was the first test of my resolve. Make sure you have some good tires, a few sharp rocks litter the narrow, winding road up the side of the mountain. It was slow going but my Camaro (yes you read that right) made it though.
The hike itself started out pretty tough for me. The trail itself is pretty well maintained. The first couple miles are mostly uphill. However there are rewards for sticking it out. There are a few overlooks that give you amazing views of the Bitterroot's sheer walls, crags, snow capped peaks, and Peterson Lake. You'll come across some streams of runoff that you'll have to hop over. You'll find some relief when you finally realize you're going downhill, only to remind yourself you'll have to hike back up to get home. I stopped about a mile short of the lake because I was done hiking up and down the hills for the day and knew I had to slug back to my car, but I plan to return. This would be an excellent multi-day hike/camp.
Traffic was pretty light for a weekend. I came across several groups of hikers, but few and far between. I really didn't see any wildlife, but quite a bit of wildflowers if you're into that!
I wouldn't recommend this trail to beginners or kids, but it's well worth the time and soreness to explore!