I did a trip with my in-laws and my 20 month old. I'll probably get flamed for this, but we make too big a deal about what the *kids* are interested in at that age...and don't factor in the age/activity level of the in-laws.
We always have park hoppers and my daughter HATED rides, so we didn't spend a ton of time in MK. But my MIL was totally unprepared for the amount of walking and by the second day was beyond done with it.
We only did 4 park days. MK-->Epcot, DHS --> Epcot, Epcot--> MK, and then AK the last day. It worked for us, but I realize it's not the ideal for everyone.