by Dave_Ayers » Fri Jan 13, 2017 11:58 am
As AT mentions, it's the climb from Florence Lake at ~7350 to Sallie Keys at 10,200 that is the issue. The cutoff trail is quite steep (+600 ft in 0.6 mi.) and the bulk of the climb north of the cutoff is in the afternoon sun and can be quite hot.
Decades ago, JMT hikers would carry their resupply into the Diamond D Ranch (now the Muir Trail Ranch) and head up to Sallie Keys to make a weekend out of it. I first did that hike with my Dad at age 10, so I know that it is doable for boys. But you'll need them to be good hikers. Also you'll want to watch for issues with the first night altitude above 10k. Note that youths are more susceptible to mountain sickness than older folks.
If the going seems too tough, there are campsites along the river between the cutoff and JMT which would allow you to do the climb the next morning if you have 3 days (or more). There are also some camp-able spots near the meadow after the climb about 1.3 mi. before the 1st Lake that can shorten the first day a bit.