Another concern for me would be what happens if there is an accident.
Then this person is injured/incapacited because she wasn't wearing a seatbelt.
She couldn't help any of the children in the van exit or care for injuries to them.
Consider this. In an emergency situation the seatbelt helps keep you in place behind the wheel. I can tell you from early experience that it's impossible to regain control of an out of control vehicke, when you can't even keep yourself behind the wheel.
Having been in a vehicle upside down at 70mph, I can tell you that if that van ever rolled or flipped, Mrs. X would be one giant projectile that could do some damage and hurt kids.
Nobody rides with me at without a seatbelt on, I don't want them in my lap if we get in a wreck.