As others have mentioned, the pool is a great place, and I love your idea of the life jackets. As a former lifeguard, I can't tell you how many parents I watched essentially just throw their kids in the water without supervision and assume the lifeguards would babysit them. One of the worst rescues we had was where I child about 2 had fallen over about two feet from mom... she was just talking away while her son was drowning beside her. So props for the lifejackets. The mini golf courses are also very very fun, and something I think even your two year old could enjoy, especially the Fantasia Golf Course.
I'd also honestly recommend DHS. There are a lot of shows to watch (Voyage of Mermaid, Lights Motor Action, Indiana Jones, Beauty and the Beast, Fantasmic, ect). Also, I know you said you don't want to take them on rides but stuff like The Great Movie ride is ok to go on. You sit all in a row. There is even a bunch of stuff like that at the MK. (I'm thinking of Pirates, Tiki Room, Jungle Cruise, Small World, Winnie the Pooh- this one there is two rows per car so you could have your two oldest in the front, Enchanted Tales with Belle is more a show than anything, Monsters Inc Laugh Floor, ect). Just something you can think about... Guess though I'm going on the assumption your children aren't the run off on their own, have to keep a handle on them at all times. My brothers and I were very mild mannered kids and we knew the rules and what would happen if we broke them so we stayed close. If you've got more of a wild child, going to the parks might not work as well. But least you 7.5 year old is big enough that I suspect she/he should be able to help you out some at that age.
I took my two DDs to Disney alone Dec 2011. Youngest was 3. We did everything except the "big" rides. Of course, three is more difficult than two, but if your 7yo is a good helper, I don't see why at MK you couldn't do Dumbo, with the 7 and 5 yo riding together and you and the 2 yo together, as well as many of the other "small" rides (Small World, Pooh, etc.). My kids love meeting characters, so just a character hunt could be fun. Taking the parks at a slower pace - get an ice cream, sit on a bench together and talk about your favorite characters. Also, my girls love visiting the lobbies of the different resorts. Riding the monorail or the boat. DHS is awesome - lots of shows! Taken at a slower pace, you take in a few shows, watch a parade, eat an ice cream, grab an early dinner and enjoy Fantasmic. Of course, as other posters have mentioned, shh! I know it is blasphemy, but there are lots of fun things to do around Orlando away from Disney and may not be as crowded. Good luck! I LOVE travelling alone with my kids - we always feel so proud of ourselves for being resourceful. 
