I just went with my DD6 and DS5. DH had to work so I left him home with DS1. The trip was a lot of fun but also a lot of work. There were definitely a lot of meltdowns over what rides we were or were not going to go on. Luckily both were 40 inches but DS is more daring than DD so there were rides he wanted to go on & she refused. Often this occurred after she said she would go on the ride and then as the time approached to ride, she would change her mind. I literally dragged her on TOT. It was so embarassing, but knew DS would have a meltdown for the rest of the day if he did not get to ride. As it turned out, she loved it & wanted to ride again. DS didn't like it so that was it for that ride. Basically, the rule was that they both had to try a ride & then if one person didn't like it we wouldn't go on it again. DS didn't want to go on Thunder Mountain, but then loved it and we ended up going on about 10 times. Bottom line is that even if you prep them in advance, there will be some meltdowns in the moment. For the 1st 2 days, I tried having them each take turns picking what ride we would go on next but that didn't work very well, I felt like someone was always upset. By the 3rd day, I just said that I would make the decisions on what we would do & when and after that we had a much better time.
I agree with the double stroller. I brought a single & thought they could switch off but they ended up fighting over it. Finally on day 2, I rented a double stroller and it was great. Hard to push and tiring for me, but it eliminated a lot of whining.
I did 2 buffets as well and they were a bit of a pain. I actually asked a family next to me to watch the kids one time as I was going up for about the 5th time to get the kids something else they wanted. I just couldn't deal with bringing them both up again. I think my point is to not be afraid to ask for help.
When using the bathroom I would make them go in the stalls on either side of me so I could see their feet and no one was allowed out until I was out of my stall. Since my kids are different sexes they both like their privacy.
I struck up lots of conversations in line with other people as I just needed some adult interaction every once in awhile and it definitely helped me keep my sanity!
Since you're the only adult you can make all the decisions which I loved. My kids stayed up later than they ever have (DH is pretty rigid with bedtimes). We had some TS meals which I cancelled at the last minute because we were having too good a time doing something else. We snuck in pool time whenever we could, even if we only had a half hour, it was always great to take a quick dip. We stayed at WL which we loved & probably will never be able to stay at again because we're a family of 5. Overall, we just kind of went with the flow, if we wanted to do someting we had planned we would do it but if I wanted to change plans at the last minute I could and did. There were some trying times, and some times when I said "why did I bother bringing them here?" but now looking back only a week later, I really only mostly remember all the fun we had. Just relax, you will have a great time!!!