Originally Posted by LapLap

Actual baby policies at Japanese hotels can be way more fluid than what is suggested by the booking portals, especially when using points. So long as the child is sharing a bed and doesn’t require any extra bedding you can contact the hotel and make sure they won’t be charging you more for occupancy, quite a few of the chains are quite clear about children staying free under these circumstances. Once confirmed, book the room for two with your certs and then email to say yoú´ll have an infant with you.
It´s different once kids are older, may as well use up those certs now.

examples:
https://www.hilton.com/en/hotels/tyohitw-hilton-tokyo/#
¨Children under 6 years old can stay free when sharing a bed with parent(s). Guests 6 years and older are considered adults. For triple and quadruple occupancy, extra bed and/or breakfast charge will be required for the additional adult.¨

https://www.tokyo.grand.hyatt.co.jp/...olicy-EN-1.pdf
¨Children under the age of 12 can stay in their parent or guardian’s room at no extra charge. If you plan to
stay as a family in one room, please inform our staff when making a reservation.¨