I've flown quite a bit with my children - including solo with children ages 3.5, 2.5 and 1.5 years old.
First of all - check the carseats - don't try to carry them on the plane. It's just going to be too much to carry on the plane with you - plus the youngest will likely be on her lap anyway. Just make sure the carseats are tagged really, really well!
2nd - bring a double stroller - you can take it right up to the gate and "gate-check" it right before getting on the airplane. Then they bring the stroller to the door of the plane like they do a wheelchair. Even if they children won't ride in it - I found it very helpful for transporting the carry-on luggege! If she doesn't have a double - at least bring a single stroller.
3rd - get safety harnesses for the children so they can't run off and get lost in the airport. It only takes a minute and it's happened to us when security was checking us out (because that thing beeped when we went through it). My ds ran off (he was only about 18 mths at the time) and they would NOT let us go after him until they were done checking and only after he was out of sight - did they send a security person after him.
4th - don't count on airplane food - bring snacks with you. I found it helpful to bring empty sippy cups along for the flight attendents to put juice into. The first time I travelled with a toddler - I didn't do this and got soaked with juice when my dd spilled the cup on my lap.
5th - carry-on's - I used a backpack for the spare clothes ( at least one outfit for everyone including myself - it's necessary - half the plane trips I've been on with toddlers - I've been puked on, pee'd on or had food/drink dumped on me) and the diapers/wipes, bag for wet/dirty clothes. Always take twice as many diapers as you think you will need. It's no fun being stranded due to flight cancellations at an airport (that doesn't sell diapers) when you are almost out of diapers. I found it more space efficint to just use disposables for the trip. Cloth take up way more room and get heavier when wet.
For toys and snacks - I bought my older dd and my ds children's little pull-on suitcases. Like the ones the flight crew pull - but child-sized They loved them and since they were on wheels - they could pull them themselves. Now they didn't always do that without complaint - which is where that double stroller came in really handy. We brought things like pads of paper, pens, stickers, a couple new books, travel sized magnadoodle, dried fruit, fruit juice gummy bears (used instead of gum for take-off and landing), granola bars, string cheese, sippy cups water bottles....
Don't count on the airline staff helping you - I've only been assisted between flights by airport staff ONCE. Every other time I've had to do it myself and even been told (very rudely) that if I can't carry everything myself - I have no business bringing that much along. That was when I was pregnant travelling with a 2 year old and 9 mth old.