Here are my tips, some are repeats, sorry... My DD just turned 5 and we went when she was 2, 3 and 4.
A well rested is a happy child. So make sure they get enough sleep. My DD never ever takes a nap at home, but she needs on at DL. In the afternoon, she just kind of puts herself to sleep in her stroller, when we aren't expecting it, and then we go back to the room. DH takes a nap with her, I get their evening clothes, replenish snacks, water, etc. and then I go shopping. DH calls me when they wake up and we meet back up. Works perfectly. We have a hard time with the time zone change, 7 am in California is 4 am here, doesn't work to our advantage at all.
A well feed child is a happy child. In case you aren't aware, you can bring snacks, water, juice, etc. into the parks. You just can't bring anything in glass or sandwich stuff where you are going to make the sandwiches there, they have to be premade. This really helps. I am constantly offering my DD, well us all, snacks, even without anyone asking. We kind of get so emersed into the whole experience we'll forgot how hungry we really are, or what time it is, so this helps us to always be fueled up. We bring, cheese sticks, granola bars, gogurts, trail mix, muffins, pretzels, sliced apples, grapes, water, juice bags, etc. I try to stay away from candy kinda things as we will be picking up those snacks in the parks. We take a little thermos soft sided cooler and put it under our stroller.
A stroller is a must. My DD never uses a stroller at home, other than to go to the zoo, even when she was 3. It is a must for us at DL. There is no way she could handle a day in the parks without. We would be carrying her around if we didn't have one. Even when we go this year she will be 5 1/2, we are still taking one. It is also a great place to stash your jackets, etc. Just do not leave anything valuable in your stroller or anything that you wouldn't mind if it was taken. Unfortunately, there have been reports of stroller thefts and things taken from strollers.
Since it is their first time, I'd pick a ride that it can be their "first" ride every time they go to DL. Make it a tradition. We did that. Our first ride is Dumbo, we ride it first on every trip. It makes for great memories. As for the rides, I agree, you must start slow, and take them at your kids pace. Once they get scared, they might be done. Start off on the nice easy rides, casey Jr., dumbo, carousel, storybook, go to toon town, etc. Let them get used to riding the rides and get comfortable and confident and progress from there. They might suprise you and have no fear. My DD at 3 loved matterhorn, just loved it, we rode it over and over again. Last nov. she loved TOT and Space Mountain, who would have thought. But I do show her the rides of
Youtube before hand so she knows what to expect, like that matterhorn, that big scary white monster with the glowing eyes as she calls him.
Take lots of pictures and use photopass. Those photopass photos might be the only ones with your whole family in them, as in our case. I treasure those photos. So we hit up as many photopass photo's we can.
Buy a silohette on main street.
Have fun, take your time, enjoy yourself, enjoy your kids and watch them discover the magic of disney. You will have tears in your eyes more than once when you see them light up with delight.