Stroller - what would it cost to rent one for the 7 days you are there? What would it cost to buy 2 more? If the cost is the same, I'd rent b/c I think the ones at WDW are sturdier, they can lay down, and the 5 year old won't be overly heavy in it, plus it probably has some storage that the umbrella ones won't. As for the one you have. If it works for you, I'd probably bring it. You'll want one in the airport and a little inconvenience getting on and off the bus 2 times per day (getting there and getting back to the hotel) is worth it, especially if it's pricy to rent one.
You don't have to pay to check a stroller. In fact, you keep the stroller with you all the way to the gate and they check it at the gate and hand it to you as soon as you get off the plane. With a 15 month old, that will be great b/c you put him in the stroller, put your bag of goodies in the stroller, have the 4 year old stand in the stand part and 5 year old has to hold on to the stroller with one hand as you walk.
For the plane change, same thing...get the stroller, load everyone up, head to the next gate. You probably won't have a bunch of extra time before you board again. I'd plan on stopping at one of the airport shops and getting a cookie for everyone or some other little snack/treat that will keep them happy for the few minutes you have to wait. You can also have the 2 older kids play I Spy or Simon Says. It's not too loud, doesn't take a lot of time, and can be done anywhere. Practice these games at home so they know how to play when you get to the airport. The 15 month old will probably just people watch.
The best thing on a plane is little things to do. If you have a dvd player that is easy to handle, that might be nice, but you'll need headphones so they an hear. You can't turn it on until you get up and you have to turn it off on the way down. On a 2 hour flight, you'll have about an hour to watch you have to decide if it's worth the hassle of setting it up (and the weight!). Do you have a leapster? Bring that. Bring colors in a box or baggie with a coloring book. I like Color Wonder markers b/c they can't get on the seat if the kids accidently drop them. If you can get a few new things, that is best. Get a calidoscope, snow globe, view finder. Try to find things that aren't too bulky so you can put a bunch of stuff in your bag. Stickers for the 15 month old to put onto a piece of paper should go a long way. Even 1 tub of play doh for the kids to squish would be fun. You don't need the extra pieces for the play doh (sizzors, shapes, etc.), just let them squish some. They will probably be pretty much into people watching for a lot of the plane time. Get some new books for them to look at. Think Dollar store or Dollar spot at Target for some ideas of things that don't have a lot of pieces but that are new and small.
I've flown with a 3 month old all the way to 10 year olds and NEVER bought ear plug things. Have a sippy cup for them to sip on in case their ears hurt, and gum if they are old enough, but I doubt they will need ear plugs.
5 essential things - wipes, snacks, drinks, toys, comfort items if the kids have them, change of clothes, diapers.
You never know if there will be a delay at the airport, so take everyone an extra set of clothes, and make sure you have snacks. Have more entertainment than you think you'll need. Snacks are huge. If you can plan the plane ride around a meal, pack sandwhiches and let the kids eat on the plane, that takes time and full bellies make for happier kids.
It's really not that bad! The only thing you didn't ask about was the car seat? Are you bringing one on the plane? If you can, the kids usually do REALLY well in their car seat b/c they are so used to it. It's another thing that if you didn't buy a plane ticket for the 15 month old, you can bring the car seat to the gate and if there is an extra seat, they will allow you to put the child in the seat. The downside is that's a lot to haul through the airport (and while you change planes), but well worth it, especially if hubby is there to help.
You MUST tell your kids what to do if they get lost. Tell them to look for someone who works there, if they can't find someone who works there, tell them to find a mommy to help them. Put your phone number on a piece of paper in their pocket with your name, their name, etc. If they get lost they need to know to hand that paper to someone. The BEST thing I ever found were some plastic bracelets from Stuck on You. They can be pre printed with a name and phone number and it goes on their wrist. It's water proof. Whenever we were in a place like WDW, Six Flags, football game, etc., I would put them on my kids and we talked about how showing it to someone would give that person Mommy's phone number so they could help find me.
Have fun!