We almost always do 4-day trips. Here's how we get the max out of the minimum.
Make the effort to get to early entry for MK; the other parks don't matter so much. I don't know how small your kids are, but if they are at least 4, consider sending them to Simba's for one evening while Mom and Dad go to dinner or to an e-night. The break will be the highlight of the trip for you, and they will love the club.
If your kids are into characters, do one character meal, pref. an early breakfast. However, it they are not into them, count your blessings and skip those autograph lines. I'm fully convinced that avoiding character Meet & Greet lines is what saves our short trips.
Take the midday breaks for naps/swims, and stay at parks until closing each night, unless the kids are having a club night.
Swim daily if at all possible. For little kids, Disney pools are marvelous places, and tend to be the among their favorite memories.
Use FP to it's maximum potential; read up on the rules and make sure you understand how the system works. However, do realize that unless you work in a parents-only e-night, most of the e-ticket rides will have to wait for another trip; extensive baby-swapping is quite time-consuming, even when using FP.
