We took our six year old DS to DL last August and since he was recently dx as being ASD, we were concerned about how the trip would go.
A few things that we found to be helpful:
Get the GaC, if you don't need it, great but if you do need it, you have it. There is no harm in getting one for the times you need it.
Let your younger child plan where you guys go. This one might be a bit more difficult for you since you have two kids but we let DS hold the map and he would pick which rides he wanted to go on. This meant that we rode Star Tours 6 times in 4 days but he really loved it.
In the
Disneyland Hotel, right next to Goofy's Kitchen and Steakhouse 55, there is a small room that plays classic Disney shorts. DS really loved going there and chilling out. (Quite literally since we were there in August and the room was nicely air conditioned!

) It was quiet and there were no other kids there at all.
Go back to your hotel for naps. Even though DS gave up his afternoon nap a few years ago, he was so exhausted that he had a nap after lunch just about every day.
I don't know how tech oriented your DS is but at 3 my little guy loved playing with our iPod and then his Nintendo DS. At 3 he had a Leapster hand held device that he really loved. We got him headphones and the rule here is still that he has to wear the headphones if he wants to play. We brought the DS to Disneyland and brought it out when he was starting to get overwhelmed. Not only did it give him something to do but the headphoens blocked out a lot of the ambient noise that bothers him.
Sunglasses, my son is sensitive to bright lights so I always had two pairs of sunglasses in case we lost one pair...and as it turned out, we almost lost them twice!
When the kids have had enough, leave. If the older boy wants to stay a bit longer, maybe one of the parents can go back to the hotel with the younger one and the other stay with the big kid?
That's all I can think of off the top of my head, good luck!