From the FAQ

Q. If I am entering the country as a family, do I have to create an account with a different email address for each family member?
A. In order to use the Service, one account is required for each person, so in principle, please create an account with a different email address for each person. However, infants and other children who cannot complete the procedures by themselves can use the Service without creating an account by registering as an “family member traveling with you”.

Refer to notes in the PDF manual (Section 3-2)
https://www.vjw.digital.go.jp/manual..._manual_en.pdf