Downieville might be a bit of a stretch since the shortest route is about 18 miles, and while none of it is exceedingly technical, the constant bumps definitely wear you out by the end of a run if you’re not used to descents that long. Also, there aren’t really “bail out” options, so once you’re going, you have to at least make it to the bottom of third divide (about 11 miles down, including the most technical bits) before you can opt for a fire road bypass. That said if your kid is stoked on spending time outside and for a challenge, it could be great!
Another option I recommend looking into is the corral network in South Lake Tahoe and/or flume trail on the Nevada side of Tahoe, both are the same distance from Sacramento as Downieville. The corral network has some of the best trails in the state and can be enjoyed by just about every level of rider (they have everything from high-alpine singletrack, to some of the rockiest-techiest stuff imaginable on Mr.Toad’s, to the best tabletop jumps I’ve had the pleasure of riding outside of a bikepark) all within a tight trail network that can be connected to the Tahoe Rim Trail. And there are shuttle services nearby if that’s what you’re into. Flume is also popular with families that ride since it’s not very technical but has incredible views of Tahoe.