Accomodation in Japan

I need a clarification on hotel room. I am travelling with my sister and nephew (3 years old) to Nagoya & Takayama. I have booked my hotel rooms at Super Hotel Nagoya & Takayama. I just found out that their double room can only accomodate 2 adults and additional person ie.kids will be counted as 1 person. In this case, i need to book a triple room instead of double. Can someone tell me that his is normal practise in Japan that 2 adults + 1 kids are consider as 3 person and you need to book a triple room in any hotels?

It seems better to find another room.
Some hotels may accept two adults and one little kid in a double room if the kid does neither use a bed nor have a hotel meal.
If a hotel does not explicitly show such a condition, a 3-year-old kid should be counted as a normal guest.

I suppose there are not so many triple rooms available in Japan (except high-class ones).
The two hotels (Super Hotel Nagoya-ekimae and Hida-Takayama) are not listing a triple room on the website (http://www.superhoteljapan.com/en/). So, even if they offer a few triple rooms, you cannot book one thorough their reservation website (https://hotel.reservation.jp/superhotel/eng/reservationtop.asp).

Results of searching an accommodation booking website (www.jalan.net/en)
: http://www.jalan.net/en/japan_hotels_ryokan/Accommodation/Aichi_Accomm...
: http://www.jalan.net/en/japan_hotels_ryokan/Accommodation/Gifu_Accommo...
(You can click the number of children and set a condition.)

It is normal practice in Europe too. For one thing average hotel bedrooms are small (Japan and Europe) so there is no room for a small bed for a child, and bed themselves are narrow, but it also has to do with fire regulations.

You can maybe book a twin.

Often the beds in single and twin rooms are 140cm semi-double sized (some hotels will give you bed sizes on their website, so you can check).

And often a child under 6 can stay for free as long as they sleep in their parent's bed.

When me, my wife, and our 4 yr old were in Japan last year, we often booked twin rooms and the two adults slept in one bed, while our daughter had the other bed. You just pay the normal twim room charge.

Try looking at some of the other business hotel chains.

Somehow, some hotels allowed kids to stay for free and some are not. I have managed to contact them and received their clarification. Thanks a lot for your help.