I don't understand. You have two sets of grandparents, you have, at least "John and Mary" and "Charles and Diana." Why would you choose only two of them?
Use their last name (family name, if it's not last) and call it "The Windsor-Mountbatten Family Reunion"
No hyphen in "reunion"
The Smith-Davis Family Reunion.
The MacDonald-Renaldi Family Reunion
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