How long it will take overall depends on a lot of factors. If you already kind of know your way around World Showcase and have a good idea of where the KidCot stations are (or at least, how to identify them), you should be able to do it, but it might be a bit rushed. Some of the stations may already have families there, and you may have to wait a bit, which you really can't control. Are your kids going to want to stay and color, or will they be content to get their stamp and go?
Do you have/are you getting the passports for your kids? I am assuming you are, because otherwise, it wouldn't really matter if you had to skip a country or two. My kids absolutely loved doing the passports, and I highly recommend them, but we had two days to get it done so we weren't concerned with time. At each station, they would want to stop and put all the stickers in after they got the stamp/message.... but if you are crunched for time, you could just have them do that part later. Although, that may contribute to the feeling of being rushed, which you are trying to avoid.
20 minutes in each country also doesn't factor in walking from one to another, stopping for snacks, bathroom breaks, et
