Night Train to Hakodate

We plan to go to Hakodate by night train. We have 3 adults and 2 children (2 and 3 years old)
1. Can we stay in 1 room in Hokutosei? Which type of room? (We prefer a room with shower)
2. Since we have JR rail pass , How much for the additional cost for Double room (3persons) on
2.1 Cassiopeia
2.2 Hokutosei
Thank you

Can we stay in 1 room in Hokutosei? Which type of room? (We prefer a room with shower)

No. Hokutosei private rooms are for either two or four people. Rooms for two people will never hold five. A compartment for four could.

The Cassiopeia, however, has compartments that hold three adults, where three adults with two children might be allowed.

Private showers are only available in the suite rooms of the Cassiopeia. Everybody else can use the public shower rooms, which are available on Hokutosei and Cassiopeia trains.

Since we have JR rail pass , How much for the additional cost for Double room (3persons) on

Basically, the children travel for free. The adults, however, will have to pay the following supplements:

For a 3-person room on the Cassiopeia: about 19,000 yen per person

On the Hokutosei: at least 12,000 yen per person