Since family boarding has nothing to do with the OP's question I'll chime in... If the child is flying internationally without both parents it is good practice to have a minor travel consent letter completed by both parents. Note, it is not required, but rather a good item to have just in case there is an issue. Here is a sample one that can be used:
https://www.us-passport-service-guide.com/minor-travel-consent-form.html
There is no official letter that must be used, but rather just have both parents acknowledge the travel is known to them in whatever format you want. The grandparents may never be questioned, but the signed letter does give them something to refer to if they do get asked about the child. Of course, the child will also need their own passport.
Hope that answers the question
--TheMiddleSeat
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