My husband and I are in our early 60s. We're fit and active. I'd seen this hike described as "easy" and good for families and children in a guidebook; however, I would definitely not take small children on this hike, and I'd be wary of taking inexperienced older children or adults as well.
The trail forks to either side of the river right about at the Boiling Springs cabin. We took the upper trail. While not intense switchbacks, it's quite narrow in places, with a steep slope to the Payette River. You go up and down over a series of hills, some of which are very steep and slippery. A portion of the trail is completely gone, due to the bad winter of 2016-17, requiring us to scramble across a slope over downed trees and slippery dirt. We were "skiing" down in squatting position to avoid falling.
If you take the lower trail along the river, you must cross the river several times, so take shoes that can get wet.
Once you get to Dash Creek (there's no sign - it's just the biggest creek you've come across) it's easier to cross the creek, I think, then keep a lookout for the actual springs. The hot spring pools are small circles where people have arranged rocks, and a pipe comes into one pool, so don't expect a big swimming hole, and you might have to dig silt out of the pool, or rearrange the rocks to make a pool.
The hike is beautiful - great views, plenty of shade, and hot water or cold water to dip into.