I've ridden on many different types of buses - not a one ever had seat belts. I've been told that chances of being injured on a bus are very small compared with the chances of being injured riding in an automobile.
As long as your children sit in the seats (no kneeling or standing), and don't stand/walk in the aisles while the bus is moving, they should be just fine. Try not to worry too much about it.
Best wishes -