This train, Aso Boy! connects Kumamoto and Beppu via Aso. When you go to Mt. Aso by a train, take the Shinkansen to Kumamoto and transfer to this train. This is the easiest way to get there.
This train offers not only access to Aso but also a big fun, especially if you travel with child(ren). This train is operated by the refurbished Kiha-183 series. This was completely redesigned by JR Kyushu chief designer, Eiji Mitooka. The facilities of this train make both parents and children enjoy the ride to Aso.
You will find how nice this train is for kids in this article.