Ok, not exactly mountain biking, but a bike that can get kids biking a bit earlier than when I started (I think I started around 7).
I went to an art show curated by a friend of mine the other day and the subject of kids and outdoor activities came up, apparently she’s got her 4 year old already biking and she told me how it was all because of a German neighbor friend she had. This neighbor taught her kid to ride a bike by first introducing them to a pedal-less bike on which the kid could learn balance without training wheels. After the kid got the balance and steering down they graduated to a full fledged regular bike, completely skipping training wheels.
At that point the neighbor passed on the bike to my friend.
So I happened to look at the REI catalog today and saw the very bike my friend told me about, it’s called a Skuut and it’s made out of wood! I thought maybe my sister might be interested in one since my nieces are 2 years old and according to the catalog the bike is for kids ages 1.5 - 3 years old “who need to learn the balance, steering and coordination necessary for bike riding”.