It's no safer than anything else you do in life, or just as safe... depends on how you look at it.
I was wary of 40 year old lap belts in my '67, so I installed some newer Silverado seats with the 3 point integrated belts, and my 4 year old son LOVES 'his' truck.
He just graduated to a booster seat, and it fits great, plus I wouldn't trade the time we get to ride around in it together for anything.
Besides, we all remember how 'safe' our parents were when we were growing up. I remember my Dad racing people in our '69 Vette while my Mom was screaming at him and I was shoved in the package try behind the seats
Shawn