If your one year old is still taking a bottle, make sure you give it to him at take off because the g-force will hurt his ears. (Same thing with landing.) If your 4 year old can drink out of a sippy cup or chew gum, that will help him.
Talk to your ped about possibly giving your 4 year old Benedryl to help him sleep. If you decide to do it, make sure you try it a week earlier to make sure it actually makes him sleepy, rather than revving him up. The last thing you want to do is to try it the first time on the plane, and then you can't keep him still. Btw, my kids asked for benedryl when we flew overseas because they hated being awake for 16 hours straight being stuck in their seats. Hours of movies seems fun at first, but that gets really old, really fast.
The airplane will turn the lights off when they want people to quiet down and rest. That's when you should consider letting them watch the ipad - not before. Little books and toys before that, and food as well. Hopefully you can find out what the children's menu is early on and order it for the kids. Bringing favorite snacks with you will help as well. The attendants can bring warm water for you for your formula. If you haven't changed from formula to milk yet, I wouldn't. Continue with the formula until you get there and get settled. The Thai milk will probably taste different from American milk, and you will want to be able to transition only once, rather than from formula to American milk to Thai milk. (Once you get in your house or apartment, buy the milk that you like the taste of and add an ounce into the formula. As your child gets used to that, add another ounce. Eventually it's mostly milk and then all milk.)
This will be hard, S., but the one thing that you CAN control is being super organized and ready for this. You will need to be right there with your kids in the "entertainment" department. You cannot allow them to think that they can get up and walk down the aisles of the plane. That will NOT work. If you start, they will expect it and cry for it, so don't even go there.
Get to the airport in plenty of time. See if someone will go with you and help you get all your luggage checked in and wait with you until you have gotten through security. Having a stroller that holds both of the kids will really help you navigate the concourses. Keep a smile on your face and talk sweet and quietly to your children. In order to hear you in the loud din around them, they will have to focus hard on you and distract them from doing things you don't like. I would buy bottled water once you get through security to use for your formula.
I always have a ziplock gallon bag in my pocket book that holds my family's passports, airline reservations, itinerary and info on my destination, as well as the customs cards. That way, I grab it out and it's all together, and then when they are done looking at it, I put it all back in my purse. In and out quick! I also say OUT LOUD (it's okay, you can laugh!) something like "passports back in the purse" to make sure I don't misplace them and my plane tickets. Believe me, I have had friends who walked away without their passports, including one of my foreign exchange students. I don't need that debacle happening in MY life, that's for sure!!
Make sure that you know what Thailand's law is regarding dates of expiration on the passports. Some countries will not let you in if they are less than 6 months away from expiration. You wouldn't have that problem with your 1 year old, but it's foreseeable with your 4 year old and you and your husband.
Have a wonderful 4 months! Avail yourself of hiring help in the house and a trusted someone to help take you around with your children to learn the culture of Thailand. You will never forget this terrific experience!!
Dawn