Disney's Magical Express will use over the road motor coaches. These coaches are high up, and have plush seating. The coaches also have storage bays underneath the coach. (think Greyhound, but nicer)
Disney Transport uses mass-transit buses. These buses are smaller, and are designed for short and quick on/off trips. Some feature a stepless entry, and a full low floor. Buses are designed to accomodate standees. (think city bus, but nicer)
Neither the motor coaches and the transit buses will have seat belts, making car seats useless. Small children can ride in thier own seat, or in the lap of thier parents.