Hi S. J. I so feel for you! I flew on the plane when my baby girl was 4 months old, and my boy was 24 months old!!!! I had all those questions you had above and, AND MORE.
first, take a deep breath. Remember that you are a mother with two small children. EVERYONE is going to recognize that and MOST of them will do everything that they can to help you. There are only the rare few (baby-haters, people who have never had any children) who are going to roll your eyes at you or not help in the slightest bit.
I think the things to think about now and prepare for are: how long are the flights - ? How long of a stretch can your older child go before needing to pee? How well does the older child listen to you? How well do the two of them get along?
If you have any behavior (not listening to you) issues, now is the time to work on them so that by the time you get on the plane, they are attuned to you and listen well when you give them direction...
If the older child is getting into that independent stage where they don't want to listen, work on creating some kind of portable, behavior management system now - I use things like stickers, rewards AFTER good behavior etc. I read another mother who tantalized her traveling children with a new toy when they sat in their airplane seat quietly and buckled up.
Talk to them ahead of time about standing in lines and staying close to you. Even your 18 month old child can be prepared mentally for the journey ahead. I let my son travel with his father on a 23 hour plane and layover journey across the world when he was 20 months old. It is possible. We described how the plane would go up up up and down down down and land and who he was going to see and then told him how after a while he would come back home...
crayons are okay attention catchers. Sometimes a new book. little hand-held toys (just one - not ones with lots of little parts) are good and if you have a backpack full of them, you can switch them out for a new one when they get bored.
re: bathroom: If the younger one is sleeping, pick them up and carry them with you while you take the older one to the bathroom. If you have to pee, take them with you. I know that may be wierd of me. There may be a small part of your brain saying, wow it is so much effort to get us all to the bathroom. But I always use any opportunity that anyone in the family has to pee, to get the other children to try. I would rather that than an accident. Usually the flight attendant can help you by holding hands with little children outside the restroom while you pee (or whatever else needs to happen in there) but that way your little one won't feel like you are deserting them in a strange situation where they don't know where you have gone. I always tell both my kids when we are traveling the #1 rule is we stick together and we all have help each other out with this. They have to wait for mommy to pee and I have to wait for them...
The good thing about layovers is that for kids it gives them a chance to get up and run around a little bit more rather than be stuck in their seats for long hours on end. I also use layovers as a chance to get a snack, and use the bathroom in a place where there is more room to maneuver with kids than those super small airplane toilets.
If you know how long the flights are beforehand, you might practice getting your children to engage in SOME ACTIVITY for that stretch of time each day (every other day etc.) and label it somehow - this is "Sitting Quietly Time" or some name so that when she/he is on the plane, they know what is expected of them there. Make the time very fun at home, (play doh, painting, reading, anything you can think of) And then you will see what sorts of activities interest them the most and you can capitalize on that when you get on the plane...
Good luck. One thing my kids always like to look at are pictures of the family. I have seen them look and look and look again at all the photos of their relatives. Or baby pictures of themselves.