If your mentioned ESF is SJS, their school bus service is coordinated by PTA. The school bus fee is charged by 4-month basis. I don't remember the fee of Route Ma On Shan. For Route Tai Wai, the charge is around $24xx/4-month.
Ma On Shan is not belong to Kln Tong school net. So they didn't have the bus route from Ma on shan to Kln Tong. I think you better looking for the private mini bus for your children.