w/ 5 day park hopper passes you should be able to manage visiting all 4 parks.
Plan to see Wishes at MK nighttime fireworks as well as the Electric Parade.
Plan to see Illuminations at EPCOT at night 9pm.
Plan to see Fantasmic! at Hollywood Studios- also nighttime spectacular!
Plan for the daytime parades at Magic Kingdom, Hollywood Studios, and Animal Kingdom- usually around 3pm.
Use fastpasses whenever possible(Peter Pan, Toy Story Mania).
W/ 2 little ones, I would suggest going early and leaving mid-afternoon for a break at the pool or a nap, and then go back @ night.
Will you be wanting to eat just quick service or table service? If you plan on table service, you MUST make reservations ASAP!
If interested in Character dining ( a bit pricey, but if you only do 1, then it's OK), then may I suggest booking Crystal Palace (Pooh characters).
Try not to waste too much time waiting in line for characters, unless you are looking for one in particular, just roam around and "get lucky" so to speak.
If your 20 month old is unable to ride, then take advantage of BABY SWAP. it's like a fastpass for couples w/ young children so that it cuts time waiting in line for rides. The best one is Splash mountain, where they have a small playground in the shade to wait.
Plan for Festival of the Lion King at Animal Kingdom.
Plan for Beauty and the Beast show at Hollywood Studios.
But, remember always no matter how SHORT your time may be in WDW, enjoy the magical memories you'll be making w/ your family!