Ugh, I've been there done this one. (thought he grandparents weren't as old as yours at the time)....
What happened, was the toddler (3 years old) got really tired on us early into our adventures, as did the grandparents. it became short days...so my recommendation would to be to plan for more than 3-4 days of adventures in disney land. also depending on the time of year, (I went there with a mildly autistic 5 year old and 2-going on three year old year old) and the crowds caused problems for them. -- LOVED standing in line, hated the actual rides and adventures. Go figure. Mom (Me) and dad (My husband) did a LOT of carrying, which taxed the old physical condition. :) THough Disneyland is geared towards lettting them run a bit without immediate adult interaction, which was a welcomed relief!
The grandparents can work to your advantage as well, if they are able to sit at exits to various adventures, and wait for the little ones, (Thus you don't have to chase them through certain things -- definetly recommend the Robinson caruso tree-house, kids of all ages just LOVE IT!) Cinderella's castle? ugh, that's gonna be a parents right behind the young one adventure, and not something older grandparents can keep up with. (Lots of stairs, and such)
I was your age when I went, and uhm, let's just say this? I was glad to be home afterwards...would I do it again now? (8 years adn 5 years) uhm...maybe...:)
good luck...just have plan A in place, and a plan B, and know Disneyland can be an energy sucking event for all! (You, the husband, grandparents, and kids). Find a hotel close, or use mass transit to get to and from disney land. build a plan to hit the highlights close to each other, (the place is HUGE! and really does take a few days to visit it all)... and don't let your little ones talk you into spending all your spare dollars in the first day. -- it's hard to resist, trust me!
Good luck.
H.R. Crystal