I have a dd5, and ds7, both have been going to DLR since 5 months old. If this is your grandsons first time, I'd get there at park opening your second day and start with the mild rides first such as It's a Small World, or Storybook Land Canal Boats, (we usually head to Alice first though), Dumbo, then Nemo and Autopia before it gets too crowded. Pick up a Fast Pass before you get on Autopia, use those later in the afternoon when it's crowded if the boys like the ride. Then Buzz then the rockets.
In the mornings we go go go and get in all the rides we can until a late lunch and visit the happiest potty on earth, then at lunch we reevaluate the crowd level. Eat either an early or late lunch to avoid standing in line which my kids have never been excited about.
Afternoon rides I'd save for when it's getting warmer and crowded, we go at a more leisurely pace since we've gotten our favorite rides out of the way in the early morning:
Monorail
Large train
Jungle Cruise
Winnie the Pooh
Pirates of Caribbean, I warn my kids about the dips and loud cannons
Main Street cars
Mark Twain riverboat and Sailing Ship Columbia
ice cream parlor and candy store
buy souvenirs
Tom Sawyer's Island - I'd save this for close to last since they may not want to leave and maybe save for the second day so that they're not so exhausted the first full day there.
Remember to save pool time at Hojo's, they'll love the pirate pool.
Rides to look up on
Youtube to see if they're too scary:
Mr. Toad's
Snow White
Pirates
Gadgets Go Coaster