I would suggest skipping at least Epcot, given the ages of your kids. The question then becomes whether Disney Junior characters are enough to get you to Hollywood Studios. Good show and good opportunity for a character meal with them, but beyond that, you'd have Toy Story Mania, Muppets 3D, Beauty & the Beast show and the Little Mermaid show. They might enjoy the stunt shows and possibly Star Tours (not a coaster, but could be a little bumpy for young ones). Your older one might even be old enough for Jedi Training Academy if there is any interest in fighting Darth Vader with a light saber. There are also other fun dining experiences there in Sci-Fi and 50's Prime Time, so you could pretty easily fill a decent portion of a day, but not sure you could justify a full day. (Some would argue the same, regardless of ages, but I love that park and can easily fill a day there, especially if going to Fantasmic at the end of the day....but might be a little too intense for the young kids.)
I would say MK for sure, and even AK, too. We have come to appreciate AK at a whole new level after really studying the map and the things to do there. Even the walking trails (Pangani and Maharajeh) that we once avoided like the plague (for no particular reason, in retrospect) have some really cool animals like the gorillas and tigers, respectively. A new baby gorilla was born in August, so that's a feature by itself that could make the trip into AK worth it. Kids at that age could also enjoy the safari, Flights of Wonder bird show, Festival of the Lion King show, Finding Nemo show (also a good place for a nap for them if they aren't into the show itself), Rafiki's Planet Watch, Triceratop Spin, character breakfast at Tusker House, and various other character greetings around that park.
And if you're able to make it work with child-swap or some other approach, some of our favorite grown-up rides are at these two parks like Rock 'n Roller Coaster, Tower of Terror, Expedition Everest and Dinosaur (if five-year old is into dinosaurs, that might be an option if not easily freaked out!).
Maybe look at the park hours in advance and plan to hit MK on the longest possible day so that you are able to take advantage of as much in that park as possible. Even if you're out late in MK, you could just get a later start the next morning in HS or AK and still hit most of the good stuff. MK has a handful of good places to let one or more of the kiddos grab a power nap (Carousel of Progress, Peoplemover, Mickey's Philharmagic, Country Bears Jamboree, WDW Railroad, etc.), so if you need to take a break or two during the day, it is fairly easily done without necessarily feeling like you have to leave the park altogether. Definitely wouldn't count out naps for the 3 and 5 year olds....getting around the parks can make for a long, tiring day! First time we took our kids, our 3 year old zonked out soon after dinner, but our 5 year old could have stayed up until midnight...she was absolutely mesmerized by the whole experience. However, she did at one point put her head on my lap and grab a quick nap on the train at one point, but that gave her what she needed to go late into the night!