Schoolbuses are safe for those children who are the size it is designed for. 50 lbs. is a guideline, it is not hard and fast. If a child is very skinny and is not 50 lbs., but is tall enough, compartmentalization will still work pretty darned well. The reason "preschool aged" children are not supposed to be unrestrained on the bus is threefold. One, they are often too small (both under 50 lbs. AND too short for the way the buses were designed to contain to work properly); two, they are younger and we all know that as we get older our bone structure gets sturdier; three, they are less likely to be able to sit properly during the trip (the same reason they shouldn't be using boosters in cars.)
At least, that is what I understand after discussing it in my tech class last week.
Of course IANAE, but I would be totally comfortable with a seven year old or ten year old who was about the same height as his peers, or even at the low end of the curve but normal, and was able to sit properly in his seat on the bus riding the bus unrestrained, even if he was under 50 lbs. :twocents: Yes, rollovers are worse for everyone, but thankfully schoolbus rollovers are very very uncommon.