ON our school minibus we have seat belts where there is a special clip that pushes down so that the seat belt fits across their shoulder and chest (not cutting round the ears /head). Hard to explain in writing but it does position the seat belt correctly for any size child or adult which is the major problem with putting children in adult seat belts. We use these seat belts for any child over the age of about 4 but for tiny ones (e.g. 2/3 year olds) we do get their car seats from home.
Don't know if this is what the coach has that your lo is going on but if you are concerned it might be worth talking to them
our school MB have the moveable clips too which means that the belt is placed across the chest instead of the throat (!!!!).
Not sure legally what the ruling is about car seats and coaches/mini buses..you would have thought that it would really be the same !!