Hi, advice from another twin mommy. As the others have said-let the kids set the pace. We have taken the twins twice now, and are going in November again. The first trip they were 8 months old. It was FANTASTIC! We also had a 14 month old nephew with us. Then last year when the twins were 2 (the week of turing 3) we took them again. While mommy didn't have the best time (twin 3 year olds and a 1 year old-very hectic), the kids LOVED it. I was not very easy on myself and wanted to push too much on naps and such, when what I really needed to do was chill out. The kids really did enjoy themselves and even did stuff I fully expected them not to, like the Haunted Mansion. My dausghter still talks about the ghost sitting on her lap. Just know their limitations, like sitting in the stroller, sitting in a restaurant, how much rest they actually need, ect. And take it at their pace. You will really enjoy it.
This year we are going with just the twins, my daycare provider is keeping the 1 year old. We are only doing it this way, so that my full attention can be on the twins and what they want to do. My dh is in Korea for a year, so it has been super tough on the twins. I want to give them some mommy/me time. So we have decided to not take the baby with us (my parents are going with me to help with the twins). We hope to go overseas next for 3 years, so this may be the twins last trip for quite some time.
ENJOY your trip.