If you haven't been since 1982, you MUST do EPCOT! In fact, in 5 days there is no reason why you can't do every WDW park.
Soarin, Test Track, M:S, Illuminations, plus the WS, all of these are must sees!
Figure 5 days, with the park hopping option (a must). Since it has been so long, you'll want to take your time and experience as much as possible, I'm sure. MK, as you said, is a 2 day park. If you want to experience all, or most of, MGM, than the shows will certainly make this a full park day. Even if you wanted an off day, you can split your time between EPCOT and AK, which you shouldn't overlook. I realize you may hear some negative things about AK, but that shouldn't deter you. Animal Kingdom is a magnificant park, the Safaris and Jungle Trek shouldn't be missed, and even if you just do it for a half day, at least get it in there. In fact, this is a bit of a drift, but I seriously believe AK may get more attendance than MGM in the following years, once EE opens at least (they weren't too far from it last year, and showed the biggest increase in attendance).
Now, with that said, I understand you have some special circumstances, with younger children and older parents. If you had to cut one park because of time and other restraints, than I would still say disregard AK (though it seems you already have!). But EPCOT is a definite.
However, there are so many ways to plan 4 WDW days, especially if you feel comfortable with hopping from park to park. I would suggest picking up the latest edition of The Unofficial Guide and perhaps even the Official Guide as well.
The matter of Sea World, IMBO (that would be In My Biased Opinion :lol: ) can be disregarded. If you are traveling with young children and your parents, I don't believe Sea World is a must see. To get the family up early to drive to the park, and attack it like WDW is just not worth it. I'm figuring you will skip Kraken with the ages of your family members, and just watching the shows all day really doesn't justify a worthwhile day for me. I apologize for Sea World fans out there in advance.
Above all, though this may seem contradictary to my somewhat hectic post, enjoy your time planning, relax when you arrive, and make the most of your visit to WDW!