A seatbelt is NOT enough. (That video is only two minutes. Do go watch it.) That little boy was killed due to seat belt failure. At 3 years old, in a booster with lap & shoulder belt, while riding with his mom in the minivan, a 78-year-old senile woman ran a red light and plowed into the side of their van, causing it to roll into a ditch. His belt failed, and he was killed on impact when he thrown from the car.
Please don't let your little ones ride while restrained by belts only. (A booster seat only elevates your child to a better height for seatbelt restraint.) Seatbelts can, and regularly do, fail during crashes. The LATCH system, with top tether if possible, provides several layers of redundancy. And please, keep your toddlers rear facing to the limit of the seat requirements.
Here's a slow-motion clip of a lap-belt-only test. That's the beginning of a playlist, with several videos following on the topic of child restraint safety.
Here's how a larger child fares in a booster with belt. (20 second clip.) An even better one is here.
Here's a forward facing, five point harness test. (20 second clips.)
And lastly, here's how a child fares in a rear facing, five-point harness. (20 second clip.)
And if you're worried about your nine-year-old, sub 80-pound son complaining about how he's too big for a car seat, show him those clips. And then pull rank. Being paraplegic is so not cool. (And I'm sure you can find YouTube videos on paraplegic life, too.)
Keep you children safe. Please.