I assumed that the OP didn't mention the relation and the kids having the same last name would have avoided the issue. I assume that's how they were able to book it in the first place.
RCI's CFAR insurance should but that's a lot of money to cover a fairly small amount. A group rate or even a refundable rate might be a better way to go.
Nothing has changed with any of the Q class ships (excluding Q+) in terms of capacity since they entered service - they all have a 100% capacity of around 4200 (full capacity of around 5000).
The Press Room section of the RCI site has some handy quick reference statistics pages: Anthem of the Seas (royalcaribbeanpresscenter.com)
I’m sure that info will be available but many folks will try to get a price drop without bothering to look at that detail or not know that you can’t get the tax portion adjusted.