If you completed registration with “Will not submit” the child’s vaccine certificate and "No" for COVID test certificate, the screen will be yellow. If the child is eligible for an exemption, the screen will stay yellow, but there is no problem entering Japan.
If a child under the age of 18 does not have a valid vaccination certificate, but is accompanied by and living with a parent/custodian who does have one and is responsible for the child's actions, the child can receive a special exemption (not required to submit negative COVID test certificate) and will be considered as having a valid vaccination certificate like the parent/custodian.