In a baby capsule they're going to be fairly well off, but we've never done anything so dramatic that we've jolted the occupants enough to cause them brain damage. I guess I take it a lot slower!
Our first actual real get it right off the road 4WD trip with the grandson was when he was 3. He loved it - watched the Wiggles the entire time!
I guess if you feel more comfortable with having them in some sort of harness or bracing then go for it, but I think it does them good to learn how to handle unexpected motion and at that age, most kids are nearly unbreakable unless you're trying to park on them.