Our son is five and going on his 8th trip next month, it will be our first with two kids ( baby will be 5 months), so I have some experience and tips!
I totally agree with the poster who encouraged bringing favorite snacks, and I would suggest juice boxes or Capri Sun waters. My DS doesn't like drinking plain water, so we have found the Capri Suns to be lifesavers in Disney. It can't really spill, he drinks it quickly through the straw, and they fit nicely in our small cooler bag with our waters. You can place a grocery order through a delivery service like Garden Grocer if you don't have a car, very easy. I also agree on the importance of a stroller! We love Joovy strollers for their smaller size and huge canopy, plus DS says its the most comfortable. He has never complained about not walking, he stands enough in lines and walking around each area. We had to buy a sit and stand for our next trip, he feels too old to sit next to his brother in a double but still wants to avoid walking.
For lines, typically DS would play or talk with other kids in line. Otherwise we play Rock Paper Scissors, eye spy, or other silly games. Last resort, to avoid a meltdown, I always have an iPod or toy in my camera bag. I try to avoid him being head down into electronics, but will d anything to avoid a meltdown. We also go on his schedule, arrive early and by lunch we are ready for a break. Lots of families can go much longer, but we find some pool time or a rest goes a long way. Take advantage of the rider swap pass for rides your child isn't tall enough for if you are interested in the bigger rides, DH and I would do that while DS napped in his stroller when he was really young.
Book ADRs for dinner on the earlier end, you'll all be ready to get to bed, or at least relax, and won't want to be bussing back after a late dinner. I'd suggest the Disney Jr breakfast at Hollywood & Vine, and fun dinners like Teppan Edo, Ohana, Chef Mickeys, or the Hoop De Doo - we found that DS does better with some entertainment when he's tired at the end of the day, rather than a quiet restaurant!