Osaka - Beppu Sunflower Ferry - with child?

Hi r/JapanTravelTips

I'm planning to take the Sunflower ferry from Osaka to Beppu sometime in April. I'm traveling with my 5 year old son, and was wondering if the ferry accommodates a twin share or some other configuration where we can share a room.

When booking online, all I've seen are single bed rooms or semi-private dorm rooms. I don't feel comfortable leaving him by himself so was wondering if anyone knew of a way to share a room?