I would have to say that the motorcycle is more dangerous. I know most people on here are older than me and have even more experience, but that's what I think. I've been riding dirt bikes for I guess 3 or 4 years now, but I've been a passenger all my life. But, these are just 100 & 125cc trail bikes. We (me, my dad, my grandfather) do sometimes take my niece on a ride, or they'll take my cousins. Both on tractors or trail bikes. I don't think anyone under 10 should ride passenger on a street bike. I think if you are safe and have been operating the same machine for a while, then yes, go ahead and take the kids on a trail bike.
Tractors, they can be dangerous if in the wrong hands. But I grew up riding them and was driving by myself at 6 or so. That was with 25hp or so tractorsm now because of that early experience, I can be trusted on almost everything, I've even driven my neighbors Harleys and his rollback, and I have a truck of my own now. I think starting them early and safely is the way to do things,
Kyle