The only thing I can add, and this is going back quite a few years, is our travel agent at the time suggested getting the
DCL cruise insurance as it was much cheaper for my Mom, who was 'getting up there' in age. I agree about the Medivac for your Grandmother and $500,000 would be more than enough for the Caribbean.
Do 7 year olds need insurance? I don't know . . . I tend to insure the ancient members of the family . . . . And check your credit cards - some of them extend insurance to 'family' members on a trip. Mine even covers nieces and nephews.
If a 7 year old gets appendicitis on the cruise, they may want to have insurance (and it's sometimes free or very low cost with coverage from the parent's travel insurance, depending on who you get the insurance from).
OR, if the 7 year old has a significant injury, illness, or other catastrophe PRIOR to the cruise that causes the booking to have to be canceled-- I guess at least someone in that kid's family needs insurance that covers it. Or the family plans to be able to absorb the cost of the cruise and doesn't mind tossing several thousand dollars out the window (what we would refer to as self-insured).
These are good questions to ask the company providing the insurance.
At the risk of sounding like a broken record-- we have had to use travel insurance when my DH had an accidental injury less than a week prior to the cruise that put him in a situation where we couldn't travel, and the entire cruise fare would have been nonrecoverable, had we not had insurance. Could we have afforded to toss $2500 out the window? Maybe, but I don't like losing money like that. So the travel insurance isn't just for illness/injury/travel snafus during the dates of your travel, but also in the dates leading up to your travel.
Within a year of that incident, another family member had an accidental injury prior to traveling, and didn't have travel insurance... they lost over $10,000. I'd be less comfortable losing that than $1500.
Another thing to consider is-- if something happens along the way on the cruise, and the ship gets you to a hospital (or clinic) at the port town, what if that facility cannot provide all the care you need (illness/injury) and you need to be transported somewhere else for definitive care? That's a good thing to have coverage for. (Alaska cruisers-- pay attention-- places like Hoonah, Sitka, and Skagway do not have major hospitals.)