score:4
According to https://www.nps.gov/seki/learn/nature/sherman.htm, there is a "Main Trail" and an "Accessible Trail". The Main Trail is 1/2 mile long. An adult walking briskly could probably do it in 10 minutes but I don't know how fast your niece walks.
The Accessible Trail is only described as "short". Based on the Google Map it appears to be about 1000 feet. However, you cannot park there unless you have a disability parking placard. You can ride a shuttle there from late May to early September.