I loved this chapter book in the 1970s.  It took place during a much earlier era.   A coal mine in, I believe, Wales is closing which employs most of the people in the town.  The town children are afraid of what will happen to the ponies that work in the mine pulling the coal carts. They want to “retire” the ponies to a field.  The parents are worried about losing their jobs and don’t have the money to help the children save the ponies.  I believe the children make an appeal to the wealthy owner of the mine, who is sympathetic.  There may also be descriptions of the miners being sick, and of a canary dying from gas in the mine.  I remember this book having the same kind of feel to it as “The Wheel on the School.”  Thanks everyone for your help!