That is a good question. Preschoolers should be in a restraint system, regardless of vehicle type. I'm surprised that any state would waive their restraint laws for 15 passenger vans, given the poor safety record of those vehicles.
In my opinion, a group of 3-year olds using only lap belts in a 15-passenger van is a horrible tradgedy waiting to happen:-(
There already WAS a tragedy just like this in SC ( where I am from) I sadly witnessed the accident. It was horrible. An 18 wheeler could not stop at a red light ( it was a blind intersection) and t-boned the van. Little kids were thrown out of the van and allover this 4 lane highway! I can still vividly remember it!!
It's been quite a few years ago but because one of the children died in the accident, SC law no longer allows daycares to use these types of vans to transport children. The children's parents enacted a new law to prevent this from happening again.
http://www.jacobs-law.org/
Here is an excerpt about the incident
"In 1994, Heathwood Hall Episcopal school of South Carolina had a 15-passenger van (owned by the school) involved in an accident - several injured, one young man died. The parents sued everybody. The driver of the van was not at fault - truck ran a red light and hit them broadside. The result was the single largest wrongful death settlement ever in the state of South Carolina. Here's why: 1. It is a violation of Federal Law (title 49) for a dealer to sell a vehicle to a school that does not meet federal Motor Vehicle safety Standards consistent with the 1974 Congressional mandate for school bus safety. The van dealer was found liable. 2. Federal regulations DO NOT prohibit the use of these vehicles once the school owns them, but the school was found negligent for using an improper vehicle to transport students."