I’m sorry I don’t know anything about traveling in Greece, but in general, if you are making vacation plans with kids, let them be involved in the planning process. Find out a bunch of options for what you all can do and let each of the kids pick at least one special thing that they want to do.
You should also make sure that you plan in plenty of downtime, where the kids can just sit around and take a nap, or go for a swim in the pool or read a book or go online to talk to their friends back home or color or work a puzzle book. Do not over-plan your activities. Even kids who are excited to go on vacation can have meltdowns if there is not some planned in down time. Especially if you have to do a lot of driving or a lot of walking, or a lot of looking at ruins which may or may not be interesting to kids. Even stuff that is interesting for kids, is less tolerable for kids than it is for parents, simply because their attention spans tend to be shorter and what they are getting out of any experience is quite different than what a parent or adult would be getting out of it.
Make sure that you always pack some snacks that you know your kids will like. Some kids might be adventurous, to a certain extent, when it comes to food, but most are not. Make sure the snacks are in your purse or backpack each day when you set out on an adventure. One thing that will spoil everybody’s good time is having a picky kid who is hungry, but who will not even try to eat any of the local delicacies. If you need goldfish and chocolate chip breakfast bars, make sure that you have enough of them for your entire trip, or find out if you can purchase them wherever you might go.
Whatever activities you decide to do, try not to do any that last more than two hours, and one hour is even better. Kids just get ancy and bored, even if they’re looking at history and art that is super-fascinating to adults.
Make sure you verbalize your expectations every time you go out of your hotel. The kids are in a new environment and everything is potentially exciting or distracting, plus they’re on vacation. You need to remind them, each time before you go out, what kind of behavior you expect of them, and what you plan to do if someone mishbehaves, whether that’s pulling them aside and having a time out or a conversation, or simply abandoning the activity and going back to the hotel.
Make sure you have access to (still don’t know what the situation is about carrying liquids onto planes these days) anti-diahrreal medication like Imodium, and anti-tummy trouble meds like Pepto Bismol, fever reducer (I think Tylenol is OK for kids, but make sure you check, because I’m not positive if that’s the right one) just in case someone comes down with something, and something like Neosporin To Go for things like bug bites and bee stings and minor cuts and scrapes, plus bandaids, and Benadryl, just in case someone has an allergic reaction to something.
Every day, before you head out, take a facial photo and a full length photo of your kid’s outfit that he’s wearing that day. If one of your kids gets lost, and you’re hysterical, you might not even be able to recall what he was wearing, but if you have a daily photo in your phone or camera, that can be very helpful. It might be a good idea to pin your name, cell phone number and the number of the American Embassy in Greece into the inside of your kid’s shirts. We made some up with a piece of fabric and a laundry marker and safety pinned them inside of my nephew’s shirt when we went camping, just in case he got lost. We also gave him a whistle (which can be worn around the neck of the older kids, who are less likely to get the string caught on something, and safety pinned onto the shirts of the younger kids, so that they are able to reach the whistle and blow it without having to un-do the safety pin, just in case they get into some kind of danger, or get lost, they should blow it.
Before you leave the US (I’m assuming that you are in the US) make sure that you photo copy your kid’s passports and keep the copies in a separate purse, bag or suitcase or safe of your hotel, than the real ones. Make another copy of the passports to keep back at home with Grandma or a neighbor, just in case something gets lost. Also make sure that Grandma also knows exactly where you’re going to be staying (address and phone number of the hotels) and also has the phone number of the American (or whichever country you are coming from, maybe UK) Embassy in Greece and that Grandma has current photos of the kids and yourselves (take them a week before you leave) just in case she needs some assistance locating you guys in case of an emergency.
Once you’ve got all of this stuff down, try to have some fun!