The Tahoe Donner trail system is unbeatable. I have hiked them, mountain biked them and have been horseback riding. They are well marked, so even if you are alone, it's difficult to get lost. The TD Association has done a great job of keeping them up, adding bridges over creeks, clearly marking the boundaries, and keeping them clear of garbage. Euer Valley is a beautiful place to bike or hike.
There are some steep climbs, but the maps clearly mark them and you can still enjoy an all day hike wth young children.