What are you bringing? My average sized 7 year old is 57 lbs, which puts him out of the weight range for every stroller on the market that I know of. We would never dream of putting him in a stroller, but maybe because he's the oldest of 4 kids. He's successfully done 3 Disney trips sans stroller, and we move pretty fast. Anyway, you can't fit a 7 year old in a small stroller, so at least one will be big and bulky. The 5 year old is also probably out of the weight range for an umbrella stroller, so I'm assuming we are talking 2 full sized strollers. If so, each is about 20 lbs plus the weight of a 5 and 7 year old, which if we are talking about average sized kids is about 42 and 55 lbs, respectively. I wouldn't worry about the buses too much if your 16 year old is willing to fold and lug it (and hold it while likely standing on the bus) because your kids are old enough to get on the bus unassisted. What I would worry about is if you and your 16 year old are physically able to push 60-80 lbs all day in the heat and humidity. Personally, I've never seen a 7 year old in a stroller, but I guess that would be my biggest concern.
We have 7, 4, 2 year olds, and a 3 month old. We will be going next month and will be bringing 2 lightweight strollers for the 2 year old and baby. If the 4 year old gets sleepy, I can probably carry the baby in the ergo and give him a short ride, but the weight limit on the stroller is 40 lbs and he's close to it so it may not work. We've ruined a stroller on a Disney trip by letting a slightly larger than recommended child use it. With all of the miles we put on in a trip, it broke the stroller, so we are not going to do that again. Rental companies won't let you go over the weight limit for that reason.
So, I guess if it's worth lugging 2 full size strollers through an airport, on buses, to restaurants etc. and carrying them/emptying them every time you get on a bus, and chasing them around all day, I would go with 2 full sized strollers.
Personally, while it's not a huge deal to do it while our kids are young, I'm counting down the years until our first stroller free trip! About 4-5 years to go!