I would only be nervous about the youngest's neck since the muscles aren't quite developed enough to support the weight of the head and thus being a little more susceptible to whiplash. Your wife has a good reason to be nervous but as I already mentioned, with the proper safety items in place I wouldn't be worried. Once they're into booster seats you won't have an issue though. I'm 20 and my dad never took me snowmobiling much until my neck muscles were strong enough to hold the helmet up. So I was 2 our 3yo