If you're going south, it's a climb for a few miles with large steps and switchbacks. It would be difficult for kids less than 10 years unless they are particularly well conditioned. Going north from this location is fairly flat for a little over a mile and there is a cool historic section with boulders that can be climbed up to a reservoir there.
I agree with all Stacie said, but I would add that my kids did the hike south a few times at ages 5 & 7 and if you: start by 9am, take some snacks, don't rush, and go to the actual top of Donner Summit it can be quite an accomplishment for the youngins. Another tip: you can just down from Rainbow Bridge and hike up to the tracks across the rock.
That depends on so many variables but yes, it is well marked and easy to follow.
Moderate grades: good hiking boots/shoes, walking stick, water & snacks, hat. Take time and enjoy views & nature.
Steep incline?
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