How to travel with kids to destination wedding

I'd just talk to your fiancé about this and make a judgement call!
I definitely think it's good to think about how you'll use those days before the wedding, and if it will even be enjoyable for them to join early!

To your question, I think it depends on how you'll be spending your time and how much you want this to be a family bonding trip vs your wedding. At 12 & 14, the kids are also old enough to have their own opinions about how they get there and how long they stay.

Also, if you haven't already, you'll want to check the rules/laws regarding minor children traveling internationally. They will likely need to have notarized letters signed by both parents in order to travel internationally (even if they travel with their dad, they'll probably need to have signed letters from their mom). I have a friend who has sole custody of her daughter, but because their last names don't match, she couldn't take the child out of the country for a planned trip without signed papers from the child's father.... (From the US Customs & Border Patrol website: Minors may be able to travel to another country without either one of their parents. They may however require a notarized written consent letter from both parents. Minors interested in traveling without their parents should contact the embassy to address admissibility questions. A list of embassies and entry requirements can be obtained at the Department of State website, or call the particular embassy by phone.)