Here are my thoughts along with some questions:
(1) You will need more food/water/washroom breaks than you probably currently envision (and each child will “have to go” at a different time. Keep this in mind when making plans. And if you decide to step into a long line for an attraction, offer a washroom opportunity to the kids before getting in that line.
(2) Are you renting strollers? I highly recommend with kids that age.
(3) What times do you plan to spend a the parks? Are you early morning rope droppers? Will you take a break in the afternoons (in the park or going back to the resort?) Will you go back to the park for evenings? Will the 2 y/o nap in the park in the stroller or does it need to be back at the resort? Do the 5 y/o nap? If not, are they the type of kids who would prefer to spend all day at the park (like my DD7 did when she was 4.5 yo) or are they more like my DS4.5 who prefers a mid-day break?
(4) Measure your kids height in advance so you have an idea of what rides they will need to skip in advance. Helps for planning your day and also helps avoid kids getting excited for a ride, only to be disappointed when they are not tall enough.
The attractions my kids were tall enough for might also influence the number of days I picked for each park along with …
(5) Do your 5 y/o’s enjoy thrill rides? Do they like Star Wars?
(6) For a onece in a lifetime trip, if it fits in your budget I would consider the MK Fireworks dessert party with the garden view of the fireworks. We did this when my kids were 4.5 y/o and 2 y/o. The garden will be crowded but much less crowded than the regular viewing area. And what kid doesn’t love free reign at a dessert buffer for 45 mins. That was one of the highlights of that trip for my kids. NOTE: I would be bring sound deadening earphones for the kids unless you know that they are ok with very loud fireworks.
(7) Since you have G+, read the G+ thread in these forums. So much valuable advice. I have been to WDW many times, but last month was my first trip with G+ and that thread helped save me a lot of time in the parks.
(8) Minor tips, but I have found these helfup: Bring disposable ponchos (multiple for each person). Bring portable battery chargers for your phone. Bring snacks for the kids to the parks (more optimal, but saves time and money). Bring post-it notes to stick over the sensor on the toilet if your kids may be scared of the loud noise of auto-flushers that could go off while they are sitting on the toilet. If you rent strollers, also get stroller rain covers and cup holders.
I would spend a full day at each park and a second full day at MK. I would lean toward doing your first partial day as a few hours at MK (stack those LLs!), but if you told me your twins were over 40", love thrill rides and love Star Wars, I could see spending that partial day at HS. The way HS works, it will be very hard to do the Star Wars Rides, the Toy Story Rides, ToT and MMRR in one day without staying all day and waiting in some long lines. Broken up into 1.5 days, you may be able to do them all with Genie+ LLs (and ILL for Rise of the Resistance) and avoid really long lines.